Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball travelling a great flight distance on driver shots, exhibiting an excellent spin performance on approach shots under a dry condition and a wet condition, and showing an excellent shot feeling and durability. The present invention provides a golf ball comprising a golf ball body having a spherical core, an intermediate layer and a cover, and a paint film composed of two or more layers, wherein a 10% elastic modulus difference (Min-Mout) between an innermost layer and an outermost layer of the paint film is 25 kgf/cm2 or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. application Ser. No. 15/381,966 filed on Dec. 16, 2016, which claims priority to Japanese Patent Application No. 2015-247257, filed on Dec. 18, 2015. The entire contents of all of the above applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a technology for improving the flight performance and spin performance of a golf ball.

DESCRIPTION OF THE RELATED ART

A golf ball traveling a great flight distance on driver shots and showing an excellent spin performance on approach shots is desirable. Thus, a golf ball having an improved flight performance on driver shots and an improved spin performance on approach shots, which is conventionally obtained by adjusting a hardness or thickness of the spherical core, intermediate layer and cover of the golf ball, has been proposed (for example, refer to claim 1, paragraphs 0010, 0011 in Japanese Patent Publication No. 2010-188199 A, and claim 1, paragraphs 0010, 0011 in Japanese Patent Publication No. 2013-031778 A).

Further, as a method of improving the performance of a golf ball on approach shots from the viewpoint of a material, a method of appropriately choosing a cover material can be exemplified. If the cover material is appropriately chosen, the spin rate on approach shots can be increased, and the performance on approach shots of 40 yards or more can be improved. For example, Japanese Patent Publication No. 2011-092328 A discloses a golf ball comprising a core and a cover, wherein the cover is formed from a golf ball resin composition containing a polyurethane, and the polyurethane includes a polyol component having a number average molecular weight in a range from 200 to 1500 as a constituent component (refer to claim 6 and paragraph 0011 in Japanese Patent Publication No. 2011-092328 A).

In addition, a paint film is formed on a surface of a golf ball body. Improving the properties of a golf ball by improving the paint film thereof has also been proposed. Japanese Patent Publications No. 2015-126772 A, No. 2015-195878 A and No. 2015-195879 A disclose a golf ball having a paint film, wherein a base resin constituting the paint film is a polyurethane obtained by a reaction between a polyol composition and a polyisocyanate composition, the polyol composition contains a urethane polyol that includes a polyether diol having a number average molecular weight in a range from 800 to 3000 as a constituent component, the paint film has a 10% elastic modulus of 100 kgf/cm2 or less, and the 10% elastic modulus (kgf/cm2) (Y) of the paint film and a molar ratio (NCO/OH) (X) of the NCO group in the polyisocyanate composition to the OH group in the polyol composition satisfy a relationship of Y≤200×X−75 (refer to claim 1, paragraphs 0035-0037 in Japanese Patent Publication No. 2015-126772 A, paragraphs 0014, 0046 in Japanese Patent Publication No. 2015-195878 A, and paragraphs 0014, 0049 in Japanese Patent Publication No. 2015-195879 A).

SUMMARY OF THE INVENTION

As mentioned above, the construction design made for the purpose of striking a good balance between the flight performance on driver shots and the spin performance on approach shots, and the cover material or paint film material chosen for the purpose of improving the spin performance on approach shots, have been proposed. However, there is still room for improvement on the spin rate on approach shots under a wet condition. The present invention has been made in view of the abovementioned circumstances, and an object of the present invention is to provide a golf ball travelling a great flight distance on driver shots, exhibiting an excellent spin performance on approach shots under a dry condition and a wet condition, and showing an excellent shot feeling and durability.

The present invention that has solved the above problems provides a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the golf ball body comprises a spherical core, an intermediate layer covering the spherical core and a cover covering the intermediate layer, and the paint film has a multi-layered construction composed of two or more layers, and wherein a difference (Min-Mout) between a 10% elastic modulus (Min) of an innermost layer of the paint film and a 10% elastic modulus (Mout) of an outermost layer of the paint film is 25 kgf/cm2 or more. The relatively soft outermost layer of the paint film increases the spin rate on approach shots under a dry condition, and the relatively hard innermost layer of the paint film increases the spin rate on approach shots under a wet condition.

In one preferable embodiment, the intermediate layer has a thickness Tm of 0.75 mm or more, and the cover has a material hardness Hc of 40 or less in Shore D hardness. If the thickness of the intermediate layer is 0.75 mm or more, the golf ball has an improved durability. If the material hardness of the cover is 40 or less in Shore D hardness, the golf ball has an improved shot feeling.

In another preferable embodiment, a hardness difference (surface hardness-center hardness) HDc (Shore C hardness) between a surface hardness and a center hardness of the spherical core, a material hardness Hm (Shore D hardness) of the intermediate layer, a thickness Tc (mm) of the cover, and a material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (1);

(HDc×Hm)/(Hc×Tc)>63   Mathematical formula (1).

If the mathematical formula (1) is satisfied, the spin rate on driver shots can be decreased.

In yet another preferable embodiment, a volume Vm (mm3) and the material hardness Hm (Shore D hardness) of the intermediate layer, a phantom volume Vc (mm3) and the material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (2);

3.0<(Vm×Hm)/(Vc×Hc)<7.5   Mathematical formula (2).

If the mathematical formula (2) is satisfied, the spin performance on approach shots can be improved while the spin rate on driver shots is suppressed.

According to the present invention, a golf ball travelling a great flight distance on driver shots, exhibiting an excellent spin performance on approach shots under a dry condition and a wet condition, and showing an excellent shot feeling and durability is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is a schematic view of an example of a contact force tester used in the present invention;

FIG. 3 is a partially enlarged cross-sectional view of a collision plate of the contact force tester;

FIG. 4 is a graph illustrating an example of Ft(t), Fn(t) and M(t);

FIG. 5 is a cross-sectional view of a groove shape of a surface layer material of the contact force tester; and

FIG. 6 is a graph illustrating an example of Ft(t), Fn(t) and M(t).

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the golf ball body comprises a spherical core, an intermediate layer covering the spherical core and a cover covering the intermediate layer, and the paint film has a multi-layered construction composed of two or more layers, and wherein a difference (Min-Mout) between a 10% elastic modulus (Min) of an innermost layer of the paint film and a 10% elastic modulus (Mout) of an outermost layer of the paint film is 25 kgf/cm2 or more.

If the outermost layer of the paint film is soft, the spin rate on approach shots under a dry condition is increased. In addition, if the innermost layer of the paint film is hard, the spin rate on approach shots under a wet condition is increased. Accordingly, the relatively soft outermost layer of the paint film and the relatively hard innermost layer of the paint film can increase the spin rate on approach shots under a wet condition while maintaining the spin rate on approach shots under a dry condition.

In one preferable embodiment, the intermediate layer has a thickness Tm of 0.75 mm or more, and the cover has a material hardness Hc of 40 or less in Shore D hardness. If the thickness Tm of the intermediate layer is 0.75 mm or more, the golf ball has an improved durability. If the material hardness Hc of the cover is 40 or less in Shore D hardness, the golf ball has an improved shot feeling, and the improvement effect of the spin rate on approach shots under a dry condition is further enhanced.

In another preferable embodiment, a hardness difference (surface hardness-center hardness) HDc (Shore C hardness) between a surface hardness and a center hardness of the spherical core, a material hardness Hm (Shore D hardness) of the intermediate layer, a thickness Tc (mm) of the cover, and a material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (1);

(HDc×Hm)/(Hc×Tc)>63   Mathematical formula (1).

The mathematical formula (1) is an index indicating a decrease degree of the spin rate on driver shots. A greater hardness difference HDc of the spherical core and a greater material hardness of the intermediate layer indicate a further decreased spin rate on driver shots. In addition, a lower material hardness of the cover and a thinner thickness of the soft cover indicate a further decreased spin rate on driver shots. Accordingly, a larger value of the ratio ((HDc×Hm)/(Hc×Tc)) indicates a further enhanced spin rate decrease effect on driver shots.

In yet another preferable embodiment, a volume Vm (mm3) and the material hardness Hm (Shore D hardness) of the intermediate layer, a phantom volume Vc (mm3) and the material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (2);

3.0<(Vm×Hm)/(Vc×Hc)<7.5   Mathematical formula (2).

The mathematical formula (2) is an index indicating an increase degree of the spin rate on approach shots under a dry condition. If the material hardness of the cover is lower than the material hardness of the intermediate layer, and the thickness of the cover is thinner than the thickness of the intermediate layer, the spin rate on approach shots under a dry condition is increased while the spin rate on driver shots is suppressed. It is noted that, if the value of the product (Vm×Hm) is excessively large, the suppression effect of the spin rate on driver shots is so great that the spin rate on approach shots under a dry condition is decreased. Accordingly, if the ratio ((Vm×Hm)/(Vc×Hc)) falls within the range from 3.0 to 7.5, the spin rate on approach shots under a dry condition can be increased while the spin rate on driver shots is suppressed.

1 Physical Properties

1.1 Paint Film

The golf ball has a paint film formed on the surface of the golf ball body. The paint film is not particularly limited, as long as the paint film has a multi-layered construction composed of two or more layers, but the paint film preferably comprises four or less layers, more preferably three or less layers, even more preferably two layers. If the paint film comprises four or less layers, the painting process does not become excessively complicated, and thus productivity is better.

The difference (Min-Mout) is 25 kgf/cm2 (2.5 MPa) or more, preferably 45 kgf/cm2 (4.4 MPa) or more, more preferably 65 kgf/cm2 (6.4 MPa) or more, and is preferably 400 kgf/cm2 (39.2 MPa) or less, more preferably 375 kgf/cm2 (36.8 MPa) or less, even more preferably 350 kgf/cm2 (34.3 MPa) or less. If the difference (Min-Mout) is less than 25 kgf/cm2, there is a poor balance between the spin performance on approach shots under a dry condition and the spin performance on approach shots under a wet condition, and if the difference (Min-Mout) exceeds 400 kgf/cm2, delamination inside the paint film may occur, and thus the paint film may have poor durability.

The 10% elastic modulus (tensile stress at 10% strain) (Min) of the innermost layer of the paint film is preferably 100 kgf/cm2 (9.8 MPa) or more, more preferably 125 kgf/cm2 (12.3 MPa) or more, even more preferably 150 kgf/cm2 (14.7 MPa) or more, and is preferably 500 kgf/cm2 (49.0 MPa) or less, more preferably 450 kgf/cm2 (44.1 MPa) or less, even more preferably 400 kgf/cm2 (39.2 MPa) or less. If the innermost layer of the paint film has a 10% elastic modulus (Min) of 100 kgf/cm2 or more, the spin performance on approach shots under a wet condition is further enhanced, and if the innermost layer of the paint film has a 10% elastic modulus (Min) of 500 kgf/cm2 or less, the paint film has better durability. It is noted that if the innermost layer of the paint film has an excessively high 10% elastic modulus, the paint film becomes so hard that crack may occur in the paint film.

The maximum elongation (strain at break) of the innermost layer of the paint film is preferably 30% or more, more preferably 40% or more, even more preferably 50% or more, and is preferably 200% or less, more preferably 175% or less, even more preferably 150% or less. If the innermost layer of the paint film has a maximum elongation of 30% or more, occurrence of crack in the paint film may be suppressed, and thus the paint film has better durability. If the innermost layer of the paint film has a maximum elongation of 200% or less, the spin performance on approach shots under a wet condition is further enhanced.

The 10% elastic modulus (Mout) of the outermost layer of the paint film is preferably 5 kgf/cm2 (0.5 MPa) or more, more preferably 10 kgf/cm2 (1.0 MPa) or more, even more preferably 15 kgf/cm2 (1.5 MPa) or more, and is preferably less than 100 kgf/cm2 (9.8 MPa), more preferably 90 kgf/cm2 (8.8 MPa) or less, even more preferably 80 kgf/cm2 (7.8 MPa) or less. If the outermost layer of the paint film has a 10% elastic modulus (Mout) of 5 kgf/cm2 or more, the paint film has better stain resistance, and if the outermost layer of the paint film has a 10% elastic modulus (Mout) of 100 kgf/cm2 or less, the spin performance on approach shots under a dry condition is further enhanced.

The maximum elongation (strain at break) of the outermost layer of the paint film is preferably 100% or more, more preferably 120% or more, even more preferably 140% or more, and is preferably 500% or less, more preferably 450% or less, even more preferably 400% or less. If the outermost layer of the paint film has a maximum elongation of 100% or more, the spin performance on approach shots under a dry condition is further enhanced and the shot feeling becomes better. If the outermost layer of the paint film has a maximum elongation of 500% or less, the paint film does not become excessively soft, and thus the stain resistance becomes better.

When the paint film comprises three or more layers, the 10% elastic modulus of the layer other than the innermost layer and the outermost layer is preferably 5 kgf/cm2 (0.5 MPa) or more, more preferably 10 kgf/cm2 (1.0 MPa) or more, even more preferably 15 kgf/cm2 (1.5 MPa) or more, and is preferably 500 kgf/cm2 (49.0 MPa) or less, more preferably 450 kgf/cm2 (44.1 MPa) or less, even more preferably 400 kgf/cm2 (39.2 MPa) or less. In addition, it is preferred that the 10% elastic modulus of the layer other than the innermost layer and the outermost layer is lower than the 10% elastic modulus of the innermost layer, and is higher than the 10% elastic modulus of the outermost layer.

The total thickness of the paint film is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 20 μm or more, and is preferably 60 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less. If the paint film has a total thickness of 10 μm or more, the spin performance on approach shots under a dry condition and a wet condition is further enhanced, and if the paint film has a total thickness of 60 μm or less, the paint film has little influence on the dimple pattern formed on the cover, and thus the flight performance becomes better. The thickness of the paint film can be obtained, for example, by measuring the cross-section of the golf ball using a microscope (VHX-1000 available from Keyence Corporation).

The thickness (Tin) of the innermost layer of the paint film is preferably 5 μm or more, more preferably 7.5 μm or more, even more preferably 10 μm or more, and is preferably 30 μm or less, more preferably 27.5 μm or less, even more preferably 25 μm or less. If the innermost layer of the paint film has a thickness (Tin) of 5 μm or more, the spin rate on approach shots under a wet condition is further increased, and if the innermost layer of the paint film has a thickness (Tin) of 30 μm or less, occurrence of crack in the paint film is suppressed, and thus the paint film has better durability.

The thickness (Tout) of the outermost layer of the paint film is preferably 5 μm or more, more preferably 7.5 μm or more, even more preferably 10 μm or more, and is preferably 30 Ξm or less, more preferably 27.5 μm or less, even more preferably 25 μm or less. If the outermost layer of the paint film has a thickness (Tout) of 5 μm or more, the spin rate on approach shots under a dry condition is further increased, and if the outermost layer of the paint film has a thickness (Tout) of 30 μm or less, the paint film has better stain resistance.

The ratio (Tout/Tin) of the thickness (Tout) of the outermost layer of the paint film to the thickness (Tin) of the innermost layer of the paint film is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.4 or more, and is preferably 5 or less, more preferably 4 or less, even more preferably 3 or less. If the ratio (Tout/Tin) is 0.2 or more, lowering in the spin rate on approach shots under a dry condition is suppressed, and if the ratio (Tout/Tin) is 5 or less, the increase effect of the spin rate on approach shots under a wet condition is further enhanced.

1.2 Golf Ball Body

The golf ball body constituting the golf ball comprises a spherical core, an intermediate layer covering the spherical core and a cover covering the intermediate layer.

In one preferable embodiment, the intermediate layer has a thickness Tm of 0.75 mm or more, and the cover has a material hardness Hc of 40 or less in Shore D hardness.

In another preferable embodiment, a hardness difference (surface hardness-center hardness) HDc (Shore C hardness) between a surface hardness and a center hardness of the spherical core, a material hardness Hm (Shore D hardness) of the intermediate layer, a thickness Tc (mm) of the cover, and a material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (1);

(HDc×Hm)/(Hc×Tc)>63   Mathematical formula (1).

In yet another preferable embodiment, a volume Vm (mm3) and the material hardness Hm (Shore D hardness) of the intermediate layer, a phantom volume Vc (mm3) and the material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (2);

3.0<(Vm×Hm)/(Vc×Hc)<7.5   Mathematical formula (2).

The ratio ((HDc×Hm)/(Hc×Tc)) of the product (HDc×Hm) obtained by multiplying the hardness difference HDc (Shore C) of the spherical core by the material hardness Hm (Shore D) of the intermediate layer to the product (Hc×Tc) obtained by multiplying the material hardness Hc (Shore D) of the cover by the thickness Tc (mm) of the cover, is more than 63, preferably 64 or more, and more preferably 65 or more. If the ratio ((HDc×Hm)/(Hc×Tc)) is more than 63, the spin rate on driver shots is decreased, and thus the golf ball travels a great flight distance. In addition, the ratio ((HDc×Hm)/(Hc×Tc)) is preferably 175 or less, more preferably 170 or less, and even more preferably 165 or less. If the ratio ((HDc×Hm)/(Hc×Tc)) is 175 or less, the excessive decrease in the spin rate on driver shots is suppressed, and thus the flight performance is enhanced.

The ratio ((Vm×Hm)/(Vc×Hc)) of the product (Vm×Hm) obtained by multiplying the volume Vm (cm3) of the intermediate layer by the material hardness Hm of the intermediate layer to the product (Vc×Hc) obtained by multiplying the volume Vc (cm3) of the cover by the material hardness Hc of the cover, is more than 3.0, preferably 3.2 or more, more preferably 3.4 or more, and is less than 7.5, preferably 7.3 or less, more preferably 7.0 or less. If the ratio ((Vm×Hm)/(Vc×Hc)) is 3.0 or less, the spin rate on driver shots is increased and thus the flight performance deteriorates, or the durability deteriorates. If the ratio ((Vm×Hm)/(Vc×Hc)) is 7.5 or more, the spin rate on approach shots under a dry condition is decreased.

The product (HDc×Hm) is preferably 1000 or more, more preferably 1100 or more, even more preferably 1200 or more, and is preferably 2800 or less, more preferably 2700 or less, even more preferably 2600 or less. If the product (HDc×Hm) is 1000 or more, the spin rate on driver shots is decreased and thus the flight performance is further enhanced. If the product (HDc×Hm) is 2800 or less, the excessive decrease in the spin rate on driver shots is suppressed, and thus the flight performance is enhanced.

The product (Hc×Tc) is preferably 7 or more, more preferably 8 or more, even more preferably 9 or more, and is preferably 22 or less, more preferably 21 or less, even more preferably 20 or less. If the product (Hc×Tc) is 7 or more, the excessive decrease in the spin rate on driver shots is suppressed and thus the flight performance is enhanced, and if the product (Hc×Tc) is 22 or less, the spin rate on driver shots is decreased and thus the flight performance is further enhanced.

The product (Vm×Hm) is preferably 260,000 or more, more preferably 270,000 or more, even more preferably 280,000 or more, and is preferably 600,000 or less, more preferably 590,000 or less, even more preferably 580,000 or less. If the product (Vm×Hm) is 260,000 or more, the golf ball has a further enhanced durability, and if the product (Vm×Hm) is 600,000 or less, the spin rate on approach shots under a dry condition is increased.

The product (Vc×Hc) is preferably 48,000 or more, more preferably 49,000 or more, even more preferably 50,000 or more, and is preferably 130,000 or less, more preferably 120,000 or less, even more preferably 110,000 or less. If the product (Vc×Hc) is 48,000 or more, the spin rate on approach shots under a dry condition is increased, and if the product (Vc×Hc) is 130,000 or less, the excessive increase in the spin rate on approach shots under a dry condition is suppressed and thus the controllability becomes better.

1.2.1 Cover

The cover is the outermost layer of the golf ball body. The cover is adjacent to the surface of the intermediate layer directly or via a reinforcing layer.

The material hardness Hc of the cover is 40 or less, preferably 39 or less, more preferably 38 or less in Shore D hardness. If the material hardness Hc of the cover is 40 or less in Shore D hardness, the spin performance on approach shots improves. The lower limit of the material hardness Hc of the cover is, without particular limitation, preferably 22 or more, more preferably 23 or more, and even more preferably 24 or more in Shore D hardness. If the material hardness Hc of the cover is 22 or more in Shore D hardness, the spin rate on driver shots is further decreased, and thus the golf ball travels a greater flight distance.

The thickness of the cover Tc is preferably 0.2 mm or more, more preferably 0.25 mm or more, even more preferably 0.3 mm or more, and is preferably 0.7 mm or less, more preferably 0.65 mm or less, even more preferably 0.6 mm or less. If the thickness of the cover Tc is 0.2 mm or more, the spin performance on approach shots becomes better, and if the thickness of the cover Tc is 0.7 mm or less, the spin rate on driver shots is decreased and thus the golf ball travels a greater flight distance. It is noted that the thickness of the cover is the thickness of the part on which no dimple is formed.

The phantom volume Vc of the cover is preferably 1100 mm3 or more, more preferably 1400 mm3 or more, even more preferably 1600 mm3 or more, and is preferably 3900 mm3 or less, more preferably 3700 mm3 or less, even more preferably 3500 mm3 or less, most preferably 3400 mm3 or less. If the phantom volume Vc of the cover is 1100 mm3 or more, the spin performance on approach shots becomes better, and if the phantom volume Vc of the cover is 3900 mm3 or less, the spin rate on driver shots is decreased and thus the golf ball travels a greater flight distance. It is noted that the phantom volume of the cover is the volume of a cover when assuming that no dimples exist thereon.

1.2.2 Intermediate Layer

The intermediate layer is a layer formed on the surface of the spherical core. The intermediate layer is adjacent to the inner side of the cover directly or via a reinforcing layer. The intermediate layer may have a single layer, or two or more layers. It is noted that, when the intermediate layer has two or more layers, the material hardness, thickness and volume of the intermediate layer disposed at the outermost side, are adopted as Hm, Tm and Vm, respectively.

The material hardness Hm of the intermediate layer is preferably 55 or more, more preferably 58 or more, even more preferably 60 or more, and is preferably 80 or less, more preferably 78 or less, even more preferably 75 or less in Shore D hardness. If the material hardness Hm of the intermediate layer is 55 or more in Shore D hardness, the spin rate on driver shots is decreased and thus the golf ball travels a greater flight distance, and if the material hardness Hm of the intermediate layer is 80 or less in Shore D hardness, the golf ball has a better shot feeling.

The thickness Tm of the intermediate layer is preferably 0.75 mm or more, more preferably 0.80 mm or more, even more preferably 0.85 mm or more, and is preferably 1.9 mm or less, more preferably 1.8 mm or less, even more preferably 1.7 mm or less. If the thickness Tm of the intermediate layer is 0.75 mm or more, the golf ball has a better durability, and if the thickness Tm of the intermediate layer is 1.9 mm or less, the spherical core has a relatively large diameter and thus the resilience performance is enhanced.

The volume Vm of the intermediate layer is preferably 3900 mm3 or more, more preferably 4200 mm3 or more, even more preferably 4400 mm3 or more, and is preferably 9500 mm3 or less, more preferably 9100 mm3 or less, even more preferably 8600 mm3 or less. If the volume Vm of the intermediate layer is 3900 mm3 or more, the golf ball has a better durability, and if the volume Vm of the intermediate layer is 9500 mm3 or less, the spherical core has a relatively large diameter and thus the resilience performance is enhanced.

The difference (Hm−Hc) between the material hardness Hm of the intermediate layer and the material hardness Hc of the cover is preferably 27 or more, more preferably 28 or more, even more preferably 29 or more, and is preferably 47 or less, more preferably 46 or less, even more preferably 45 or less in Shore D hardness. If the difference (Hm−Hc) is 27 or more in Shore D hardness, the spin performance on approach shots becomes better, and if the difference (Hm−Hc) is 47 or less in Shore D hardness, the spin rate on driver shots is decreased and thus the golf ball travels a greater flight distance.

The ratio (Tm/Tc) of the thickness Tm of the intermediate layer to the thickness Tc of the cover is preferably 1.3 or more, more preferably 1.4 or more, even more preferably 1.5 or more, and is preferably 4.0 or less, more preferably 3.9 or less, even more preferably 3.8 or less. If the ratio (Tm/Tc) is 1.3 or more, the golf ball has a better durability, and if the ratio (Tm/Tc) is 3.9 or less, the spin performance on approach shots becomes better.

The ratio (Vm/Vc) of the volume Vm of the intermediate layer to the phantom volume Vc of the cover is preferably 1.2 or more, more preferably 1.3 or more, even more preferably 1.4 or more, and is preferably 3.5 or less, more preferably 3.4 or less, even more preferably 3.3 or less. If the ratio (Vm/Vc) is 1.2 or more, the golf ball has a better durability, and if the ratio (VmNc) is 3.5 or less, the spin performance on approach shots becomes better.

1.2.3 Spherical Core

The spherical core may have a single-layered construction, or a multi-layered construction composed of two or more layers. In light of the easy hardness design, a dual-layered core composed of a spherical center and an outer core covering the spherical center is preferred.

The hardness difference (surface hardness-center hardness) HDc between the surface hardness and the center hardness of the spherical core is preferably 15 or more, more preferably 16 or more, even more preferably 17 or more, and is preferably 40 or less, more preferably 39 or less, even more preferably 38 or less in Shore C hardness. If the hardness difference HDc is 15 or more in Shore C hardness, the spin rate on driver shots is decreased and thus the golf ball travels a greater flight distance.

The center hardness of the spherical core is preferably 45 or more, more preferably 46 or more, even more preferably 47 or more, and is preferably 70 or less, more preferably 69 or less, even more preferably 68 or less in Shore C hardness. If the center hardness is 45 or more in Shore C hardness, the golf ball has an enhanced resilience, and if the center hardness is 70 or less in Shore C hardness, the spin rate on driver shots is further decreased.

The surface hardness of the spherical core is preferably 75 or more, more preferably 76 or more, even more preferably 77 or more, and is preferably 90 or less, more preferably 89 or less, even more preferably 88 or less in Shore C hardness. If the surface hardness is 75 or more in Shore C hardness, the spin rate on driver shots is further decreased, and if the surface hardness is 90 or less in Shore C hardness, the spherical core does not become excessively hard and thus the durability thereof becomes better.

The diameter of the spherical core is preferably 37.5 mm or more, more preferably 37.7 mm or more, and even more preferably 38.0 mm or more. If the diameter of the spherical core is 37.5 mm or more, the resilience of the spherical core becomes better. The upper limit of the diameter of the spherical core is, without particular limitation, preferably 41.5 mm or less, more preferably 41.0 mm or less, and even more preferably 40.5 mm or less.

When the spherical core has a diameter in a range from 37.5 mm to 41.5 mm, the compression deformation amount (shrinking amount along the compression direction) of the spherical core when applying a load from 98 N as an initial load to 1275 N as a final load to the spherical core is preferably 2.3 mm or more, more preferably 2.5 mm or more, and is preferably 4.5 mm or less, more preferably 4.2 mm or less. If the compression deformation amount is 2.3 mm or more, the shot feeling becomes better, and if the compression deformation amount is 4.5 mm or less, the resilience becomes better.

1.2.4 Reinforcing Layer

The golf ball body may further comprise a reinforcing layer between the intermediate layer and the cover. The reinforcing layer firmly adheres to the intermediate layer, and at the same time, firmly adheres to the cover. The reinforcing layer suppresses delamination of the cover from the intermediate layer.

In particular, when a golf ball having a thin cover is hit with the edge of the club face, wrinkle easily occurs. The wrinkle is suppressed by the reinforcing layer. The reinforcing layer has a thickness of 20 μm or less.

2 Material

2.1 Paint Film

Examples of the base resin constituting the paint film include a urethane resin, an epoxy resin, an acrylic resin, a vinyl acetate resin, and a polyester resin. Among them, the urethane resin is preferred. In the case that the base resin constituting the paint film is the urethane resin, tensile properties of the paint film can be adjusted by the formulation of the polyol composition or polyisocyanate composition, or the mixing ratio thereof. It is noted that the base resin constituting each layer of the paint film may be different from each other, but it is preferred that the base resin constituting all layers of the paint film is the urethane resin.

(Polyurethane Paint)

The paint film is preferably formed from a paint containing a polyol composition and a polyisocyanate composition. Examples of the paint include a so-called two-component curing type urethane paint containing a polyol as a base material and a polyisocyanate as a curing agent.

(Polyol Composition)

The polyol composition contains a polyol compound. The polyol compound is a compound having at least two hydroxyl groups in the molecule thereof. Examples of the polyol compound include a compound having a hydroxyl group at the terminal of the molecule, and a compound having a hydroxyl group at a part other than the terminal of the molecule. The polyol compound may be used solely or as a mixture of at least two of them.

Examples of the compound having a hydroxyl group at the terminal of the molecule include a low molecular weight polyol having a molecular weight of less than 500 and a high molecular weight polyol having a number average molecular weight of 500 or more. Examples of the low molecular weight polyol include a diol such as ethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; and a triol such as glycerin, trimethylolpropane, and hexanetriol. Examples of the high molecular weight polyol include a polyether polyol, a polyester polyol, a polycaprolactone polyol, a polycarbonate polyol, a urethane polyol, and an acrylic polyol. Examples of the polyether polyol include polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG). Examples of the polyester polyol include polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA). Examples of the polycaprolactone polyol include poly-ε-caprolactone (PCL). Examples of the polycarbonate polyol include polyhexamethylene carbonate.

The urethane polyol is a compound having a plurality of urethane bonds in the molecule thereof, and having at least two hydroxyl groups in one molecule. Examples of the urethane polyol include a urethane prepolymer obtained by a reaction between a first polyol component and a first polyisocyanate component, under a condition that the hydroxyl group of the first polyol component is excessive to the isocyanate group of the first polyisocyanate component.

Examples of the first polyol component constituting the urethane polyol include a polyether diol, a polyester diol, a polycaprolactone diol, and a polycarbonate diol, but the polyether diol is preferred. Examples of the polyether diol include polyoxyethylene glycol, polyoxypropylene glycol, and polyoxytetramethylene glycol. Among them, polyoxytetramethylene glycol is preferred.

The number average molecular weight of the polyether diol is preferably 600 or more, more preferably 650 or more, even more preferably 700 or more, and is preferably 3000 or less, more preferably 2500 or less, even more preferably 2000 or less. If the number average molecular weight of the polyether diol is 600 or more, the distance between crosslinking points in the paint film becomes long and the paint film becomes soft, thus the spin performance is enhanced. If the number average molecular weight of the polyether diol is 3000 or less, the distance between crosslinking points in the paint film does not become excessively long, and thus the stain resistance of the paint film becomes better. The number average molecular weight of the polyol component can be measured, for example, by gel permeation chromatography (GPC), using polystyrene as a standard material, tetrahydrofuran as an eluate, and an organic solvent system GPC column (e.g., “Shodex (registered trademark) KF series” available from Showa Denko K. K.) as a column.

The first polyol component may include a low molecular weight polyol having a molecular weight of less than 500. Examples of the low molecular weight polyol include a diol such as ethyleneglycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; and a triol such as glycerin, trimethylolpropane, and hexanetriol. The low molecular weight polyol may be used solely or as a mixture of at least two of them.

The first polyol component constituting the urethane polyol preferably includes a triol component and a diol component. As the triol component, trimethylolpropane is preferred. The mixing ratio (triol component/diol component) of the triol component to the diol component is preferably 0.2 or more, more preferably 0.5 or more, and is preferably 6.0 or less, more preferably 5.0 or less in a mass ratio.

The first polyisocyanate component constituting the urethane polyol is not limited, as long as the first polyisocyanate component has at least two isocyanate groups. Examples of the first polyisocyanate component include an aromatic polyisocyanate such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylenediisocyanate (TMXDI), and para-phenylene diisocyanate (PPDI); an alicyclic polyisocyanate or aliphatic polyisocyanate such as 4,4′-dicyclohexylmethane diisocyanate (H12MDI), hydrogenated xylylenediisocyanate (H6XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI). The first polyisocyanate may be used solely or as a mixture of at least two of them.

The amount of the polyether diol in the urethane polyol is preferably 70 mass % or more, more preferably 72 mass % or more, and even more preferably 75 mass % or more. The polyether diol forms a soft segment in the paint film. Therefore, if the amount of the polyether diol is 70 mass % or more, the obtained golf ball has further enhanced spin performance.

The weight average molecular weight of the urethane polyol is preferably 5,000 or more, more preferably 5,300 or more, even more preferably 5,500 or more, and is preferably 20,000 or less, more preferably 18,000 or less, even more preferably 16,000 or less. If the weight average molecular weight of the urethane polyol is 5,000 or more, the distance between crosslinking points in the paint film becomes long and the paint film becomes soft, thus the spin performance is enhanced. If the weight average molecular weight of the urethane polyol is 20,000 or less, the distance between crosslinking points in the paint film does not become excessively long, and thus the stain resistance of the paint film becomes better.

The hydroxyl value of the urethane polyol is preferably 10 mg KOH/g or more, more preferably 15 mg KOH/g or more, even more preferably 20 mg KOH/g or more, and is preferably 200 mg KOH/g or less, more preferably 190 mg KOH/g or less, even more preferably 180 mg KOH/g or less. The hydroxyl value can be measured according to JIS K 1557-1, for example, by an acetylation method.

Examples of the compound having a hydroxyl group at a part other than the terminal of the molecule include a modified polyrotaxane having a hydroxyl group and a hydroxyl group-modified vinyl chloride-vinyl acetate copolymer.

The modified polyrotaxane having a hydroxyl group has a cyclodextrin, a linear molecule inserted into the cyclic structure of the cyclodextrin, and having blocking groups located at both terminals of the linear molecule to prevent disassociation of the cyclic molecule. The polyrotaxane is viscoelastic, since the cyclodextrin molecule is movable along the linear molecule that penetrates the cyclodextrin in a skewering manner (pulley effect). Even if a tension is applied to the polyrotaxane, the tension can be uniformly dispersed due to the pulley effect. Thus, the polyrotaxane has an excellent property that a crack or flaw very hardly occurs, unlike a conventional crosslinked polymer.

The cyclodextrin is a general term for an oligosaccharide having a cyclic structure. The cyclodextrin is, for example, a molecule having 6 to 8 D-glucopyranose residues being linked in a cyclic shape via an α-1,4-glucoside bond. Examples of the cyclodextrin include α-cyclodextrin (number of glucose units: 6), β-cyclodextrin (number of glucose units: 7), and γ-cyclodextrin (number of glucose units: 8), and α-cyclodextrin is preferable. As the cyclodextrin, one type may be used solely, and two or more types may be used in combination.

The linear molecule is not particularly limited, as long as it is a linear molecule capable of keeping the cyclic structure of the cyclodextrin in a skewering manner so that the cyclic structure of the cyclodextrin is movable along and rotatable around the linear molecule. Examples of the linear molecule include polyalkylene, polyester, polyether, and polyacrylic acid. Among them, polyether is preferable, and polyethylene glycol is particularly preferable. Polyethylene glycol has less steric hindrance, and thus can easily keep the cyclic structure of the cyclodextrin in a skewering manner.

The weight average molecular weight of the linear molecule is preferably 5,000 or more, more preferably 6,000 or more, and is preferably 100,000 or less, more preferably 80,000 or less.

The linear molecule preferably has functional groups at both terminals thereof. When the linear molecule has the functional group, the linear molecule can easily react with the blocking group. Examples of the functional group include a hydroxyl group, carboxyl group, amino group, and thiol group.

The blocking groups are not particularly limited, as long as they are located at both terminals of the linear molecule to prevent the cyclodextrin from disassociating from the linear molecule. Examples of the method for preventing the disassociation include a method of using a bulky blocking group to physically prevent the disassociation, and a method of using an ionic blocking group to electrostatically prevent the disassociation. Examples of the bulky blocking group include a cyclodextrin and an adamantyl group. The number of the cyclodextrins kept by the linear molecule (keeping amount) preferably ranges from 0.06 to 0.61, more preferably ranges from 0.11 to 0.48, and even more preferably ranges from 0.24 to 0.41, if the maximum keeping amount is deemed as 1. This is because if the keeping amount is less than 0.06, the pulley effect may not be exerted, and if the keeping amount exceeds 0.61, the cyclodextrins are so densely located that the movability of the cyclodextrin may decrease.

As the polyrotaxane, a polyrotaxane having at least a part of hydroxyl groups of the cyclodextrin being modified with a caprolactone chain, is preferred. This is because if at least a part of hydroxyl groups of the cyclodextrin of the polyrotaxane is modified with the caprolactone, steric hindrance between the polyrotaxane and the polyisocyanate is alleviated, so that the efficiency of a reaction with the polyisocyanate increases.

As the above modification, for example, the hydroxyl groups of the cyclodextrin are treated with propylene oxide to hydroxylpropylate the cyclodextrin, and then ε-caprolactone is added to perform ring-opening polymerization. As a result of this modification, the caprolactone chain —(CO(CH2)5O)nH (n is a natural number of 1 to 100) is linked to the exterior side of the cyclic structure of the cyclodextrin via —O—C3H6-O— group. “n” represents the degree of polymerization, and is preferably a natural number of 1 to 100, more preferably a natural number of 2 to 70, and even more further preferably a natural number of 3 to 40. At another terminal of the caprolactone chain, a hydroxyl group is formed through the ring-opening polymerization. The hydroxyl group at the terminal of the caprolactone chain can react with the polyisocyanate.

The ratio of the hydroxyl groups modified with the caprolactone chain to all the hydroxyl groups (100 mole %) included in the cyclodextrin before the modification is preferably 2 mole % or more, more preferably 5 mole % or more, and even more preferably 10 mole % or more. If the ratio of the hydroxyl groups modified with the caprolactone chain falls within the above range, the hydrophobicity of the polyrotaxane increases, and the reactivity with the polyisocyanate increases.

The hydroxyl value of the polyrotaxane is preferably 10 mg KOH/g or more, more preferably 15 mg KOH/g or more, even more preferably 20 mg KOH/g or more, and is preferably 400 mg KOH/g or less, more preferably 300 mg KOH/g or less, even more preferably 220 mg KOH/g or less, particularly preferably 180 mg KOH/g or less. If the hydroxyl value of the polyrotaxane falls within the above range, the reactivity with the polyisocyanate increases, and thus the durability of the paint film becomes more favorable.

The total molecular weight of the polyrotaxane is preferably 30,000 or more, more preferably 40,000 or more, even more preferably 50,000 or more, and is preferably 3,000,000 or less, more preferably 2,500,000 or less, even more preferably 2,000,000 or less, in a weight average molecular weight. If the weight average molecular weight is 30,000 or more, the paint film has sufficient strength, and if the weight average molecular weight is 3,000,000 or less, the paint film has sufficient flexibility and thus approach performance of the golf ball increases. It is noted that the weight average molecular weight of the polyrotaxane can be measured, for example, by gel permeation chromatography (GPC) using polystyrene as a standard substance, tetrahydrofuran as an eluant, and an organic solvent system GPC column (e.g., “Shodex (registered trademark) KF series” available from Showa Denko K. K.) as a column.

Specific examples of the polyrotaxane modified with the polycaprolactone include SeRM super polymer SH3400P, SH2400P, and SH1310P available from Advanced Softmaterials Inc.

The hydroxyl group modified vinyl chloride-vinyl acetate copolymer can adjust the adhesion of the paint film while maintaining the scuff resistance of the paint film. The hydroxyl group modified vinyl chloride-vinyl acetate copolymer is obtained, for example, by a method of copolymerizing vinyl chloride, vinyl acetate and a monomer having a hydroxyl group (e.g., polyvinyl alcohol, hydroxyalkyl acrylate), or by a method of partially or completely saponifying a vinyl chloride-vinyl acetate copolymer.

The amount of the vinyl chloride component in the hydroxyl group modified vinyl chloride-vinyl acetate copolymer is preferably 1 mass % or more, more preferably 20 mass % or more, even more preferably 50 mass % or more, and is preferably 99 mass % or less, more preferably 95 mass % or less. Specific examples of the hydroxyl group modified vinyl chloride-vinyl acetate copolymer include Solbin (registered trademark) A, Solbin A L, and Solbin TA3 available from Nissin Chemical Industry Co., Ltd.

Preferable embodiments of the polyol composition are an embodiment containing a urethane polyol, wherein the urethane polyol includes a polyether diol having a number average molecular weight in a range from 600 to 3000 as a constituent component (embodiment 1); and an embodiment containing a polyrotaxane, wherein the polyrotaxane has a cyclodextrin, a linear molecule inserted into the cyclic structure of the cyclodextrin, and blocking groups located at both terminals of the linear molecule to prevent disassociation of the cyclodextrin, and at least a part of hydroxyl groups of the cyclodextrin is modified with a caprolactone chain via —O—C3H6-O— group (embodiment 2).

In the embodiment 1, the amount of the urethane polyol in the polyol component of the polyol composition is preferably 60 mass % or more, more preferably 70 mass % or more, and even more preferably 80 mass % or more. It is also preferred that the polyol component of the polyol composition of the embodiment 1 consists of the urethane polyol.

In the embodiment 2, the amount of the polyrotaxane in the polyol component of the polyol composition is preferably 10 mass % or more, more preferably 15 mass % or more, even more preferably 20 mass % or more, and is preferably 100 mass % or less, more preferably 90 mass % or less, even more preferably 85 mass % or less.

The polyol composition of the embodiment 2 preferably contains a polycaprolactone polyol. The mass ratio (polycaprolactone polyol/polyrotaxane) of the polycaprolactone polyol to the polyrotaxane is preferably 0/100 or more, more preferably 5/95 or more, even more preferably 10/90 or more, and is preferably 90/10 or less, more preferably 85/15 or less, even more preferably 80/20 or less.

The polyol composition of the embodiment 2 preferably contains the hydroxyl group modified vinyl chloride-vinyl acetate copolymer. The amount of the hydroxyl group modified vinyl chloride-vinyl acetate copolymer in the polyol component of the polyol composition is preferably 4 mass % or more, more preferably 8 mass % or more, and is preferably 50 mass % or less, more preferably 45 mass % or less.

(Polyisocyanate Composition)

Next, the polyisocyanate composition will be described. The polyisocyanate composition contains a polyisocyanate compound. Examples of the polyisocyanate compound include a compound having at least two isocyanate groups.

Examples of the polyisocyanate compound include an aromatic diisocyanate such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODD, xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), para-phenylene diisocyanate (PPDI); an alicyclic diisocyanate or aliphatic diisocyanate such as 4,4′-dicyclohexylmethane diisocyanate (H12MDI), hydrogenated xylylenediisocyanate (H6XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI); and a triisocyanate such as an allophanate-modified product, a biuret-modified product, an isocyanurate and an adduct of the above diisocyanate. The polyisocyanate may be used solely or as a mixture of at least two of them.

The allophanate-modified product is, for example, a triisocyanate obtained by further reacting a diisocyanate with a urethane bond formed through a reaction between a diisocyanate and a low molecular weight diol. The adduct is a triisocyanate obtained through a reaction between a diisocyanate and a low molecular weight triol such as trimethylolpropane or glycerin. The biuret-modified products is, for example, a triisocyanate having a biuret bond and represented by the following chemical formula (1). The isocyanurate of diisocyanate is, for example, a triisocyanate represented by the following chemical formula (2).

In the formulae (1) and (2), R represents a residue where the isocyanate group is removed from the diisocyanate.

Preferable examples of the triisocyanate include an isocyanurate of hexamethylene diisocyanate, a biuret-modified product of hexamethylene diisocyanate, and an isocyanurate of isophorone diisocyanate.

In the present invention, the polyisocyanate composition preferably contains a triisocyanate compound. The amount of the triisocyanate compound in the polyisocyanate component of the polyisocyanate composition is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more. It is most preferable that the polyisocyanate component of the polyisocyanate composition consists of the triisocyanate compound.

The amount (NCO %) of the isocyanate group of the polyisocyanate contained in the polyisocyanate composition is preferably 0.5 mass % or more, more preferably 1 mass% or more, even more preferably 2 mass % or more, and is preferably 45 mass % or less, more preferably 40 mass% or less, even more preferably 35 mass % or less. It is noted that the amount (NCO %) of the isocyanate group of the polyisocyanate can be represented by 100×[mole number of isocyanate group in polyisocyanate×42 (molecular weight of NCO)]/[total mass (g) of polyisocyanate].

Specific examples of the polyisocyanate include Burnock D-800, Burnock DN-950, and Burnock DN-955 available from DIC corporation; Desmodur N75MPA/X, Desmodur N3300, Desmodur L75 (C), and Sumidur E21-1 available from Sumika Bayer Urethane Co., Ltd.; Coronate HX, and Coronate HK available from Nippon Polyurethane Industry Co., Ltd.; Duranate 24A-100, Duranate 21S-75E, Duranate TPA-100, and Duranate TKA-100 available from Asahi Kasei Chemicals Corporation; and VESTANAT T1890 available from Degussa Co., Ltd.

In the curing reaction of the curing type paint composition, the molar ratio (NCO group/OH group) of the isocyanate group (NCO group) of the curing agent to the hydroxyl group (OH group) of the base material is preferably 0.1 or more, and more preferably 0.2 or more. If the molar ratio (NCO group/OH group) is less than 0.1, the curing reaction may become insufficient. Further, if the molar ratio (NCO group/OH group) is too large, the amount of the isocyanate group is excessive, and the obtained paint film may become hard and fragile as well as the appearance of the obtained paint film may deteriorate. Thus, the molar ratio (NCO group/OH group) is preferably 1.6 or less, more preferably 1.5 or less, and even more preferably 1.4 or less. The reason why the appearance of the obtained paint film deteriorates if the amount of the isocyanate group in the paint becomes excessive is that an excessive amount of the isocyanate group may promote a reaction between the moisture in the air and the isocyanate group, thereby generating a lot of carbon dioxide gas.

When the polyol composition of the embodiment 1 is used as the polyol composition, the polyisocyanate composition preferably contains the biuret-modified product of hexamethylene diisocyanate, the isocyanurate of hexamethylene diisocyanate, and the isocyanurate of isophorone diisocyanate. In the case that the biuret-modified product of hexamethylene diisocyanate and the isocyanurate of hexamethylene diisocyanate are used in combination, the mass ratio (biuret-modified product/isocyanurate) preferably ranges from 20/40 to 40/20, and more preferably ranges from 25/35 to 35/25.

When the polyol composition of the embodiment 2 is used as the polyol composition, the polyisocyanate composition preferably contains the isocyanurate of hexamethylene diisocyanate.

The paint may be either a waterborne paint mainly containing water as a dispersion medium or a solvent-based paint mainly containing an organic solvent as a dispersion medium, but is preferably the solvent-based paint. In the case of the solvent-based paint, examples of the preferable solvent include toluene, isopropyl alcohol, xylene, methylethyl ketone, methylisobutyl ketone, ethylene glycol monomethyl ether, ethylbenzene, propylene glycol monomethyl ether, isobutyl alcohol, and ethyl acetate. The solvent may be added in either of the polyol composition and the polyisocyanate composition, and in light of uniformly performing the curing reaction, the solvent is preferably added in the polyol composition and the polyisocyanate composition, respectively.

The paint preferably further includes a modified silicone. If the modified silicone is included as a leveling agent, unevenness of the coated surface can be reduced, and thus a smooth coated surface can be formed on the surface of the golf ball. Examples of the modified silicone include a modified silicone having an organic group being introduced to a side chain or a terminal of a polysiloxane skeleton, a polysiloxane block copolymer obtained by copolymerizing a polyether block and/or a polycaprolactone block, etc. with a polysiloxane block, and a modified silicone having an organic group being introduced to a side chain or a terminal of the polysiloxane block copolymer. The polysiloxane skeleton or the polysiloxane block is preferably linear, and examples thereof include dimethyl polysiloxane, methylphenyl polysiloxane, and methyl hydrogen polysiloxane. Examples of the organic group include an amino group, epoxy group, mercapto group, and carbinol group. In the present invention, as the modified silicone oil, a polydimethylsiloxane-polycaprolactone block copolymer is preferably used, and a terminal carbinol-modified polydimethylsiloxane-polycaprolactone block copolymer is more preferably used. This is because these block copolymers have excellent compatibility with the caprolactone-modified polyrotaxane and the polycaprolactone polyol. Specific examples of the modified silicone used in the present invention include DBL-C31, DBE-224, and DCE-7521 available from Gelest, Inc.

A conventionally known catalyst can be employed for the curing reaction. Examples of the catalyst include a monoamine such as triethyl amine and N,N-dimethylcyclohexylamine; a polyamine such as N,N,N′,N′-tetramethylethylene diamine and N,N,N′,N″,N″-pentamethyldiethylene triamine; a cyclic diamine such as 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) and triethylene diamine; a tin catalyst such as dibutyl tin dilaurate and dibutyl tin diacetate. These catalysts may be used solely, or two or more of the catalysts may be used in combination. Among them, the tin catalyst such as dibutyl tin dilaurate and dibutyl tin diacetate is preferred, and in particular, dibutyl tin dilaurate is preferably used.

The paint may further include additives generally included in the paint for a golf ball, such as a filler, ultraviolet absorber, antioxidant, light stabilizer, fluorescent brightener, anti-blocking agent, leveling agent, slip agent, and viscosity modifier, where necessary.

Next, the method of applying the curing type paint composition of the present invention will be described. The method of applying the curing type paint composition is not limited, a conventionally known method can be adopted, and examples thereof include a spray coating and electrostatic coating.

In the case of performing the spray coating with an air gun, the polyol composition and the polyisocyanate composition are fed with respective pumps and continuously mixed with a line mixer located in the stream line just before the air gun, and the obtained mixture is air-sprayed. Alternatively, the polyol composition and the polyisocyanate composition are air-sprayed respectively with an air spray system provided with a device for controlling the mixing ratio thereof. The paint application may be conducted by spraying the paint one time or overspraying the paint multiple times.

The curing type paint applied to the golf ball body can be dried, for example, at a temperature ranging from 30° C. to 70° C. for 1 hour to 24 hours to form the paint film.

2.2 Golf Ball Body

2.2.1 Spherical Core

The spherical core may use a conventionally known rubber composition (hereinafter simply referred to as “core rubber composition” occasionally), and may be formed by heat pressing, for example, a rubber composition containing a base rubber, a co-crosslinking agent and a crosslinking initiator.

As the base rubber, particularly preferred is a high cis-polybutadiene having a cis-bond in a proportion of 40 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more in view of its advantageous resilience.

As the co-crosslinking agent, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms or a metal salt thereof is preferable, and a metal salt of acrylic acid or a metal salt of methacrylic acid is more preferable. As the metal constituting the metal salt, zinc, magnesium, calcium, aluminum or sodium is preferable, and zinc is more preferable. The amount of the co-crosslinking agent to be used is preferably 20 parts by mass or more and 50 parts by mass or less, with respect to 100 parts by mass of the base rubber. In the case that the aα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used as the co-crosslinking agent, a metal compound (e.g. magnesium oxide) is preferably used in combination.

As the crosslinking initiator, an organic peroxide is preferably used. Specific examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butyl peroxy)-3, 3,5-trimethylcyclohexane, 2, 5-dimethyl-2, 5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Among them, dicumyl peroxide is preferably used. The amount of the crosslinking initiator to be used is preferably 0.2 part by mass or more, more preferably 0.3 part by mass or more, and is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, with respect to 100 parts by mass of the base rubber.

In addition, the core rubber composition may further contain an organic sulfur compound. As the organic sulfur compound, diphenyl disulfides (e.g. diphenyl disulfide, bis(pentabromophenyl)persulfide), thiophenols or thionaphthols (e.g. 2-thionaphthol) may be preferably used. The amount of the organic sulfur compound is preferably 0.1 part by mass or more, more preferably 0.3 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, with respect to 100 parts by mass of the base rubber.

The core rubber composition may further contain a carboxylic acid and/or a salt thereof. As the carboxylic acid and/or the salt thereof, a carboxylic acid having 1 to 30 carbon atoms and/or a salt thereof is preferred. As the carboxylic acid, an aliphatic carboxylic acid (fatty acid) or an aromatic carboxylic acid (e.g. benzoic acid) may be used. As the fatty acid, a saturated fatty acid or an unsaturated fatty acid may be used, but the saturated fatty acid is preferred. Examples of the saturated fatty acid include carpylic acid (octanoic acid), pelargonic acid (nonanoic acid), caprin acid (decanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid. As the metal constituting the metal salt, zinc, magnesium, calcium, aluminum and sodium are preferred, and zinc is more preferred. It is noted that the fatty acid and/or the metal salt thereof excludes the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof used as the co-crosslinking agent.

The amount of the fatty acid and/or the metal salt thereof is preferably 1 part by mass or more and 40 parts by mass or less with respect to 100 parts by mass of the base rubber. It is noted that the surface of zinc acrylate which is used as the crosslinking agent is sometimes treated with zinc stearate to improve the dispersibility thereof in the rubber. In the case that zinc acrylate whose surface has been treated with zinc stearate is used, the amount of zinc stearate which is the surface treatment agent is included in the amount of the fatty acid and/or the metal salt thereof.

The core rubber composition may further contain a weight adjusting agent such as zinc oxide and barium sulfate, an antioxidant (such as dibutylhydroxytoluene), or a colored powder, in addition to the base rubber, the co-crosslinking agent, the crosslinking initiator, and the organic sulfur compound. The molding conditions for heat pressing the core rubber composition may be determined appropriately depending on the rubber formulation. Generally, the heat pressing is preferably carried out at 130° C. to 200° C. for 10 to 60 minutes, or carried out in a two-step heating of heating at 130° C. to 150° C. for 20 to 40 minutes followed by heating at 160° C. to 180° C. for 5 to 15 minutes.

2.2.2 Intermediate Layer

Examples of the intermediate layer material include a thermoplastic resin such as a polyurethane resin, an ionomer resin, a polyamide resin, and polyethylene; a thermoplastic elastomer such as a styrene elastomer, a polyolefin elastomer, a polyurethane elastomer, and a polyester elastomer; and a cured product of a rubber composition. Herein, examples of the ionomer resin include a product obtained by neutralizing at least a part of carboxyl groups in a copolymer composed of ethylene and an α,β-unsaturated carboxylic acid with a metal ion; and a product obtained by neutralizing at least a part of carboxyl groups in a terpolymer composed of ethylene, an α,β-unsaturated carboxylic acid and an α,β-unsaturated carboxylic acid ester with a metal ion. The intermediate layer may further contain a weight adjusting agent such as barium sulfate and tungsten, an antioxidant, a pigment, and the like.

Specific examples of the intermediate layer material include an ionomer resin having a trade name of “Himilan (registered trademark)” and an ethylene-methacrylic acid copolymer having a trade name of “Nucrel (registered trademark)”, both available from Du Pont-Mitsui Polychemicals Co., Ltd.; a thermoplastic polyurethane elastomer having a trade name of “Elastollan (registered trademark)” available from BASF Japan Ltd.; a thermoplastic polyamide elastomer having a trade name of “Pebax (registered trademark)” available from Arkema Inc.; a thermoplastic polyester elastomer having a trade name of “Hytrel (registered trademark)” available from Du Pont-Toray Co., Ltd.; and a thermoplastic styrene elastomer having a trade name of “Rabalon (registered trademark)” and a thermoplastic polyester elastomer having a trade name of “Primalloy (registered trademark)”, both available from Mitsubishi Chemical Corporation. The intermediate layer material may be used solely, or two or more of the intermediate layer materials may be used in combination.

The intermediate layer may be formed by injecting the intermediate layer composition directly onto the spherical core.

2.2.3 Cover

The cover material constituting the cover is not particularly limited, and examples thereof include an ionomer resin, polyurethane, polyamide, polyester and polystyrene. The polyurethane and ionomer resin are preferred. In particular, in light of the spin performance on approach shots and the abrasion resistance, the polyurethane is suitable.

The polyurethane may be either a thermoplastic polyurethane or a thermosetting polyurethane. The thermoplastic polyurethane is a polyurethane exhibiting plasticity by heating and generally means a polyurethane having a linear chain structure of a high-molecular weight to a certain extent. On the other hand, the thermosetting polyurethane (two-component curing type polyurethane) is a polyurethane obtained by polymerization through a reaction between a low-molecular weight urethane prepolymer and a curing agent (chain extender) when molding the cover. The thermosetting polyurethane includes a polyurethane having a linear chain structure or a polyurethane having a three-dimensional crosslinked structure depending on the number of the functional group of the prepolymer or the curing agent (chain extender) to be used. As the polyurethane, the thermoplastic elastomer is preferable.

Examples of the ionomer resin include a product obtained by neutralizing at least a part of carboxyl groups in a binary copolymer composed of an olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with a metal ion, a product obtained by neutralizing at least a part of carboxyl groups in a ternary copolymer composed of an olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acid ester with a metal ion, or a mixture of those. The olefin preferably includes an olefin having 2 to 8 carbon atoms. Examples of the olefin are ethylene, propylene, butene, pentene, hexene, heptene and octene. The olefin more preferably includes ethylene. Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonic acid. Among these, acrylic acid and methacrylic acid are particularly preferred. In addition, examples of the α,β-unsaturated carboxylic acid ester include methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid or the like. In particular, acrylic acid ester and methacrylic acid ester are preferable. Among these, the ionomer resin preferably includes a metal ion-neutralized product of a binary copolymer composed of ethylene and (meth)acrylic acid and a metal ion-neutralized product of a ternary copolymer composed of ethylene, (meth)acrylic acid and (meth)acrylic acid ester.

Specific examples of the cover material include an ionomer resin having a trade name of “Himilan (registered trademark)” available from Du Pont-Mitsui Polychemicals Co., Ltd.; a thermoplastic polyurethane elastomer having a trade name of “Elastollan (registered trademark)” available from BASF Japan Ltd.; a thermoplastic polyamide elastomer having a trade name of “Pebax (registered trademark)” available from Arkema Inc.; a thermoplastic polyester elastomer having a trade name of “Hytrel (registered trademark)” available from Du Pont-Toray Co., Ltd.; and a thermoplastic styrene elastomer having a trade name of “Rabalon (registered trademark)” and a thermoplastic polyester elastomer having a trade name of “Primalloy (registered trademark)”, both available from Mitsubishi Chemical Corporation. The cover material may be used solely, or two or more of the cover materials may be used in combination.

The cover may contain a pigment component such as a white pigment (e.g. titanium oxide), a blue pigment and a red pigment, a weight adjusting agent such as calcium carbonate and barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer (e.g. hindered amine light stabilizer), a fluorescent material or a fluorescent brightener, or the like, in addition to the above resin component, as long as they do not impair the performance of the cover.

The embodiment for molding the cover from the cover composition is not particularly limited, and examples thereof include an embodiment comprising injection molding the cover composition directly onto the core; and an embodiment comprising molding the cover composition into hollow shells, covering the core with a plurality of the hollow shells and compression molding the core with a plurality of the hollow shells (preferably an embodiment comprising molding the cover composition into half hollow-shells, covering the core with two of the half hollow-shells and compression molding the core with two of the half hollow-shells). After the cover is molded, the obtained golf ball body is ejected from the mold, and as necessary, the golf ball body is preferably subjected to surface treatments such as deburring, cleaning, and sandblast. If desired, a mark may be formed.

The total number of the dimples formed on the cover is preferably 200 or more and 500 or less. If the total number of the dimples is less than 200, the dimple effect is hardly obtained. If the total number exceeds 500, the dimple effect is hardly obtained because the size of the respective dimples is small. The shape (shape in a plan view) of the formed dimple includes, for example, without limitation, a circle; a polygonal shape such as a roughly triangular shape, a roughly quadrangular shape, a roughly pentagonal shape, and a roughly hexagonal shape; and another irregular shape. The shape may be employed solely, or two or more of the shapes may be employed in combination.

In the present invention, the ratio of the total area of all the dimples to the surface area of a phantom sphere is referred to as an occupation ratio. The phantom sphere is a golf ball (spherical body) when assuming that no dimples exist thereon. In the golf ball of the present invention, the occupation ratio of the dimples is preferably 60% or more, more preferably 63% or more, even more preferably 66% or more, and is preferably 90% or less, more preferably 87% or less, even more preferably 84% or less. If the occupation ratio is too high, the contribution of the paint film to the coefficient of friction becomes small. In addition, if the occupation ratio is too low, the flight performance may be lowered.

It is noted that the area of the dimple refers to the area enclosed in the periphery of the dimple when observing the central point of the golf ball from infinity. In the case of a circular dimple, the area S of the circular dimple is calculated according to the following mathematical formula.

S=(Di/2)2·π (Di: diamter of the dimple)

2.2.4 Reinforcing Layer

The reinforcing layer is formed from a reinforcing layer composition containing a resin component. As the resin component, a two-component curing type thermosetting resin is suitably used. Specific examples of the two-component curing type thermosetting resin include an epoxy resin, urethane resin, acrylic resin, polyester resin and cellulose resin. In light of the strength and durability of the reinforcing layer, the two-component curing type epoxy resin and the two-component curing type urethane resin are preferred.

The reinforcing layer composition may further contain additives such as a coloring agent (e.g. titanium dioxide), phosphoric acid-based stabilizer, antioxidant, light stabilizer, fluorescent brightener, ultraviolet absorber, and antiblocking agent. The additives may be added into the base material or curing agent of the two-component curing type thermosetting resin.

3. Golf Ball

The golf ball according to the present invention is not particularly limited, as long as it is a golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body. The construction of the golf ball body is not particularly limited, and the golf ball body may be a three-piece golf ball composed of a single-layered core, an intermediate layer covering the single-layered core and a cover covering the intermediate layer; a four-piece golf ball composed of a dual-layered core, an intermediate layer covering the dual-layered core and a cover covering the intermediate layer; a four-piece golf ball composed of a single-layered core, a dual-layered intermediate layer covering the single-layered core and a cover covering the dual-layered intermediate layer; a five-piece golf ball composed of a dual-layered core, a dual-layered intermediate layer covering the dual-layered core and a cover covering the dual-layered intermediate layer; and the like.

FIG. 1 is a partially cutaway cross-sectional view of a golf ball 20 according to one embodiment of the present invention. The golf ball 20 comprises a spherical core 21, an intermediate layer 22 covering the spherical core 21, a cover 23 covering the intermediate layer 22, and a paint film 24 formed on a surface of the cover 23. The paint film 24 comprises an inner layer 24 a and an outer layer 24 b. On the surface of the cover 23, a plurality of dimples 25 are formed. On the surface of the cover 23, a part other than the dimples 25 is a land 26.

The golf ball preferably has a diameter in a range from 40 mm to 45 mm. In light of satisfying a regulation of US Golf Association (USGA), the diameter is preferably 42.67 mm or more. In light of prevention of the air resistance, the diameter is preferably 44 mm or less, and more preferably 42.80 mm or less. The golf ball preferably has a mass of 40 g or more and 50 g or less. In light of obtaining greater inertia, the golf ball more preferably has a mass of 44 g or more, and even more preferably 45.00 g or more. In light of satisfying a regulation of USGA, the golf ball preferably has a mass of 45.93 g or less.

When the golf ball has a diameter in a range from 40 mm to 45 mm, the compression deformation amount (shrinking amount along the compression direction) of the golf ball when applying a load from 98 N as an initial load to 1275 N as a final load to the golf ball is preferably 2.0 mm or more, more preferably 2.2 mm or more, and is preferably 4.0 mm or less, more preferably 3.5 mm or less. If the compression deformation amount is 2.0 mm or more, the golf ball does not become excessively hard and thus the shot feeling thereof becomes better. On the other hand, if the compression deformation amount is 4.0 mm or less, the resilience of the golf ball becomes better.

The above golf ball comprises a golf ball body and a paint film formed on a surface of the golf ball body, and preferably has a coefficient of friction under a dry condition of 0.35 or more and 0.60 or less calculated using a contact force tester.

In the present invention, the coefficient of friction calculated using the contact force tester is a coefficient of friction between the golf ball and a collision plate when the golf ball is allowed to collide with the collision plate disposed inclined at a predetermined angle to the flying direction of the golf ball. By using the contact force tester, a time function Fn(t) of contact force in a direction perpendicular to the collision plate and a time function Ft(t) of contact force in a direction parallel to the collision plate are concurrently measured, and a maximum value of a time function M(t) which is a ratio of Ft(t) to Fn(t) represented by the following equation is defined as a coefficient of friction.

M(t)=Ft(t)/Fn(t)

In the present invention, the method of calculating the coefficient of friction will be described based on FIG. 2 to FIG. 4. FIG. 2 is a contact force tester for measuring the coefficient of friction. FIG. 3 is an enlarged cross-sectional view of a collision plate 4 that the golf ball is allowed to collide with.

The contact force tester 1 makes pseudo conditions of hitting a golf ball with a club face, and enables to measure various forces at that time. The contact force tester 1 includes, for example, a launcher 5 launching a golf ball 2 in an upward and perpendicular direction, and a collision plate 4 positioning on the upper side of the launched golf ball 2 and having a striking face 3 that the golf ball 2 collides with.

Since a distance between the launcher 5 and the striking face 3 is relatively short, an initial velocity of the golf ball 2 corresponds to a collision velocity. This collision velocity corresponds to a head speed of a club head in an actual golf swing. In view of this point, the collision velocity of the golf ball 2 to the striking face 3 may be set, for example, within the range of about 10 m/s to about 50 m/s. In the present invention, in light of the head speed of approach shots, the initial velocity is set to 19 m/s.

The desired value of the initial velocity of the golf ball 2 is set by the volume of a controller 6 or the like. Based on a distance between a first sensor S1 and a second sensor S2 provided in the launcher 5 and a time difference between interrupting these sensors, the controller 6 calculates the actually measured value of the initial velocity of the golf ball 2, and outputs the value to a computer device PC or the like.

FIG. 3 shows a partially enlarged cross-sectional view of the collision plate 4. The collision plate 4 can incline the striking face 3 at a predetermined angle a to the launching direction (flying direction) of the golf ball 2. In the present invention, an angle θ obtained by subtracting the angle a from 90 degree is defined as a collision angle. This collision angle θ corresponds to a loft angle of a club face (not shown) in an actual swing. Further, in view of the loft angle of a golf club, the collision angle θ is set to a plurality of values (e.g. 15°, 20°, 35° and the like), for example, within a range from 10° to 90°, and the measurement of the contact force, which will be described later, can be conducted at each angle. In the present invention, the collision angle θ is set to 55° in order to recreate the spin rate on approach shots.

The collision plate 4 comprises, for example, a base plate 4 a formed from a metal plate material, a superficial plate 4 c constituting the striking face 3, and a pressure sensor 4 b interposed therebetween, which are fixed to one another with a bolt 4 d integrally.

The base plate 4 a may be formed from any material without particular limitation, as long as it has a predetermined strength and rigidity, but preferably formed from steel. The base plate 4 a preferably has a thickness in a range from 5.0 mm to 20.0 mm. A model number of the main bolt 4 d is, for example, M10 according to JIS.

As the pressure sensor 4 b, for example, a 3-component force sensor is preferably used. Such sensor can measure, at least, a perpendicular force Fn in a direction perpendicular to the striking face 3, and a shear force Ft in a direction parallel to the striking face 3 (a direction from the sole side toward the crown side in a club face) as time-series data. The measurement of the force is conducted by connecting a charge amplifier or the like to the pressure sensor 4 b.

As the pressure sensor 4 b, a variety of products may be used, for example, a 3-component force sensor (model 9067) available from Kistler Instrument Corporation can be used. This sensor enables to measure force components in a parallel direction, a Y direction and a perpendicular direction. Although not illustrated, the measurement of the pressure is conducted by connecting a charge amplifier (model 5011B available from Kistler Instrument Corporation) to the pressure sensor 4 b. The pressure sensor 4 b is formed in its center with a through-hole through which the main bolt 4 d is inserted to integrally fix the pressure sensor 4 b to the base plate 4 a.

The superficial plate 4 c is composed of a main body 4 c 1 and a superficial material 4 c 2 disposed outside of the main body 4 c 1 to provide the striking face 3 and having an area large enough to collide with the golf ball 2. These are fixed with a bolt or the like, which is not illustrated, in a detachable manner. Accordingly, by appropriately changing the material, planner shape and/or surface structure of the superficial material 4 c 2, it is possible to create approximate models of various kinds of club faces and to measure the contact force thereof.

The main body 4 c 1 may be formed from any material without limitation, but typically formed from stainless steel (SUS-630). The thickness of the main body 4 c 1 is typically in a range from 10 mm to 20 mm. Further, the main body 4 c 1 may have a substantially same planner shape as the pressure sensor 4 b, for example, a square shape with a length of 40 mm to 60 mm on one side. Into the main body 4 c 1, the front end of the main bolt 4 d is screwed. As a result, the pressure sensor 4 b is interposed between the base plate 4 a and the main body 4 c 1, and the position thereof is fixed.

The superficial material 4 c 2 providing the striking face 3 of the collision plate 4 may adopt various materials, planner shapes and surface structures, however, it is preferably formed from the same material as the face (not shown) of the golf club head which has been determined as an analysis subject beforehand. In the present invention, in view of evaluating a model of approach shots, SUS-431 stainless steel which is the same material as the head material of CG-15 available from Cleveland Golf is used as the superficial material 4 c 2. The thickness of the superficial material 4 c 2 may be arbitrarily changed, for example, within a range of 1.0 mm to 5.0 mm. The planner shape of the superficial material 4 c 2 may be substantially the same as that of the main body 4 c 1, for example, a square shape with a length of 40 mm to 60 mm on one side.

The contact force tester 1 comprises a strobe device 7 and a high speed type camera device 8 enabling to take a photograph of the collision between the golf ball 2 and the striking face 3 as well as the golf ball 2 rebounding from the striking face 3. The strobe device 7 is connected to a strobe power 9. The camera device 8 is connected to a camera power 10 via a capacitor box. The imaged data is memorized in the computer device PC or the like. By comprising these devices, a slipping velocity and a contact area at the time of the collision between the golf ball 2 and the striking face 3, and a launch speed, a launch angle and a backspin rate of a golf ball, which will be explained later, can be measured.

FIG. 4 shows a time history of the perpendicular force Fn and the shear force Ft applied to the striking face 3 at the time of the collision by the golf ball 2 measured with the contact force tester 1 under a specific condition.

FIG. 4 is a graph illustrating an example of Fn(t) and Ft(t) measured with the tester shown in FIGS. 2 and 3. In FIG. 4, a point P0 represents a point where the pressure sensor 4 b starts sensing force, and generally corresponds to the point at which the striking face 3 and the golf ball 2 come into collision with each other. Fn(t) which is a contact force in the perpendicular direction gradually increases from the point P0, peaks at a point P4, and comes down therefrom to reach zero at a point P3. The point P3 represents a point where the pressure sensor 4 b no longer senses force, and generally corresponds to the point where the golf ball 2 leaves the striking face 3.

On the other hand, the value of Ft(t) which is a contact force (i.e., shear force) in the direction parallel to the collision plate increases with time from the point P0, peaks at a point P1, and comes down therefrom to reach zero at a point P2 after which it takes a negative value. Since the golf ball leaves the pressure sensor 4 b at the point P3, the curve of Ft(t) sensed by the pressure sensor 4 b takes zero at the point P3. An area S1 of the region where Ft(t) takes a positive value within the region surrounded by the curve of Ft(t) and the time axis represents impulse where the shear force is positive. On the other hand, an area S2 of the region where Ft(t) takes a negative value within the region surrounded by the curve of Ft(t) and the time axis represents impulse where the shear force is negative. Impulse S1 acts in a direction promoting back spin, and impulse S2 acts in a direction inhibiting back spin. Here, impulse S1 takes a larger value than impulse S2, and a value obtained by subtracting impulse S2 from impulse S1 contributes to back spin of a golf ball.

A coefficient of friction can be obtained by calculating a maximum value of M(t) which is expressed by Ft(t)/Fn(t).

In the present invention, the coefficient of friction under a dry condition obtained as described above is preferably 0.35 or more, more preferably 0.37 or more, even more preferably 0.39 or more, and is preferably 0.60 or less, more preferably 0.56 or less, even more preferably 0.54 or less. If the coefficient of friction under a dry condition falls within the above range, the spin rate on approach shots under a dry condition is further increased.

EXAMPLES

Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the examples described below, and various changes and modifications without departing from the gist of the present invention are included in the scope of the present invention.

[Evaluation Method]

(1) Core Hardness (Shore C Hardness)

The hardness measured at the surface of the core was adopted as the surface hardness of the core. In addition, the core was cut into two hemispheres to obtain a cut plane, and the hardness at the central point of the cut plane was measured. It is noted that the hardness at four points on the surface of the core was measured, and the average value thereof was adopted as the surface hardness. The hardness was measured with an automatic hardness tester (Digitest II, available from Bareiss company) using a testing device of “Shore C”.

(2) Material Hardness (Shore D Hardness and Shore A Hardness)

Sheets with a thickness of about 2 mm were produced by injection molding the intermediate layer composition or the cover composition. The sheets were stored at 23° C. for two weeks. At least three of these sheets were stacked on one another so as not to be affected by the measuring substrate on which the sheets were placed, and the hardness of the stack was measured with an automatic hardness tester (Digitest II, available from Bareiss company) using a testing device of “Shore D” or “Shore A”.

(3) Tensile Properties of Paint Film

The tensile properties of the paint film were measured according to JIS K7161 (2014). Specifically, the base agent and the curing agent were blended to prepare a paint, and the obtained paint was dried and cured at 40 ° C. for 4 hours to prepare a paint film (thickness: 0.05 mm). The paint film was punched out to prepare a test piece according to the test piece type 2 (width of parallel part: 10 mm, gauge length: 50 mm) prescribed in JIS K7127 (1999). The tensile test of the test piece was conducted using a precision universal tester (Autograph (registered trademark) available from Shimadzu Corporation) under testing conditions of a length between grips: 100 mm, a tensile speed: 50 mm/min and a testing temperature: 23° C.

(4) Compression Deformation Amount (mm)

The deformation amount along the compression direction (shrinking amount along the compression direction) of the golf ball, when applying a load from 98 N as an initial load to 1275 N as a final load to the golf ball, was measured.

(5) Durability

An air gun was used to allow twelve golf balls to collide with a metal plate one by one in a speed of 45 m/second, and the collision was repeated until the golf ball was broken. The collision times were accounted, and the durability was evaluated according to the following evaluation standard.

G: no crack occurred when the collision was repeated 100 times.

P: crack occurred when the collision was repeated less than 100 times.

(6) Spin rate, ball initial velocity and flight distance on driver shots (W #1)

A driver (Product name: Z745, Shaft hardness: S, loft angel: 8.5°, available from Dunlop Sports Limited) was installed on a swing robot available from Golf Laboratories, Inc. The golf ball was hit at a head speed of 50 m/sec, and the initial velocity (m/s), spin rate (rpm) and flight distance (the distance from the launch point to the stop point (yd)) of the golf ball right after hitting the golf ball were measured. This measurement was conducted twelve times for each golf ball, and the average value thereof was adopted as the measurement value for the golf ball. It is noted that a sequence of photographs of the hit golf ball were taken for measuring the spin rate right after hitting the golf ball.

(7) Measurement of Coefficient of Friction

The contact force tester shown in FIG. 2 was used to measure the coefficient of friction of the golf ball.

(7-1). Specification of Tester

(A) Launcher: Air Gun System

(B) Collision Plate:

-   -   Base plate 4 a         -   Steel         -   Thickness: 5.35 mm     -   Superficial plate 4 c         -   Main body 4 c 1             -   Size: 56 mm×56 mm×15 mm             -   Stainless steel (SUS-630)         -   Superficial material 4 c 2             -   Size: 56 mm×56 mm×2.5 mm             -   Metal composition: SUS-431             -   Groove shape: see FIG. 5     -   Angle of inclination (a)         -   35 degrees (to flying direction of golf ball)

(C) Pressure Sensor 4 b

3-component force sensor (model 9067) available from Kistler Instrument Corporation

Charge Amplifier

Model 5011B available from Kistler Instrument Corporation

(D) Capture of Contact Force into PC

A pulse counter board PCI-6101 (available from Interface Corporation) was used. With a 16-bit PCI pulse counter board having 4 channels, measurement suited for a specific application may be realized in four counter modes. The maximum input frequency is 1 MHz.

As shown in FIG. 5, the groove structure of a sand wedge CG-15 available from Cleveland Golf is reproduced on the striking face 3 of the collision plate 4. As shown in FIG. 5(a), on the striking face 3, large grooves (zip grooves) are formed, and a plurality of small grooves are formed on the surface between the large grooves (zip grooves). FIG. 5(b) is an enlarged view of cross-section structure of the zip groove. The dimensions of the zip groove are as follow.

Zip groove (groove) width W: 0.70 mm

Zip groove (groove) depth h: 0.50 mm

Zip groove (groove) pitch: 3.56 mm

Zip groove (groove) angle α: 10°

Zip groove shoulder R: 0.25

A plurality of small grooves between the zip grooves are formed by a laser-milling method such that the surface portion between the zip grooves has a surface roughness Ra=2.40±0.8 μm and Rmax=14.0±8 μm. It is noted that the surface roughness Ra and Rmax are measured by using SJ-301 available from Mitsutoyo Corporation under the conditions of specimen length=2.5 mm and cut off value=2.5 mm.

(7-2) Measuring Procedure

The coefficient of friction was measured according to the following method. The measurement temperature was 23° C. and the ball initial velocity was 19 m/s. The measurement was conducted twelves times for each golf ball, and the average value thereof was adopted as the measurement value of that golf ball. It is noted that the dry condition means that the measurement was conducted in a state that the collision plate and the golf ball were dry, and the wet condition means that the measurement was conducted in a state that the collision plate and the golf ball were wet with water.

(a) Setting the angle (α) of the collision plate at 35 degrees to the flying direction (vertical direction) of the golf ball;

(b) Adjusting the air pressure of the launcher 5;

(c) Launching the golf ball from the launcher at a speed of 19 m/s;

(d) Measuring the initial velocity of the golf ball from the preset distance between the sensor S1 and sensor S2 and the time difference between the times for the golf ball to interrupt the sensors S1 and S2;

(e) Measuring the contact force Fn(t) and contact force Ft(t), and calculating the maximum value of Ft(t)/Fn(t); and

(f) Measuring the spin rate of the golf ball with the strobe device and the camera device.

(7-3) Result of Measurement

One example of the results obtained with the above tester in the above measuring procedure is shown in FIG. 6. From FIG. 6, the value of M(t) was calculated as Ft(t)/Fn(t), and the maximum value thereof was 0.58. Since noise tends to generate in initial period where the contact force rises up and in terminal period for measuring Ft and Fn, the maximum value of M(t) was calculated after trimming an early stage of the initial period and late stage of the terminal period.

(8) Spin Rate on Approach Shots (AP) (Dry Condition and Wet Condition)

A sand wedge (CG 15 forged wedge (52°) available from Cleveland Golf) was installed on a swing robot available from Golf Laboratories, Inc. The golf ball was hit at a head speed of 21 m/s, and the spin rate (rpm) thereof was measured by continuously taking a sequence of photographs of the hit golf ball. The measurement was conducted ten times for each golf ball, and the average value thereof was adopted as the spin rate. It is noted that the dry spin rate (Sd) is a spin rate measured in a state that the club face and the golf ball were dry, and the wet spin rate (Sw) is a spin rate measured in a state that the club face and the golf ball were wet with water.

(9) Shot Feeling

An actual hitting test was carried out by ten amateur golfers (high skilled persons) using a sand wedge (CG 15 forged wedge (52°) available from Cleveland Golf). In accordance with the number of people who answered the shot feeling was good (feeling like that the golf ball was lifted on the club face, feeling like that the golf ball gripped on the club surface, feeling like that the spin was imparted, feeling like that the golf ball was stuck on the club face, etc.), the golf balls were evaluated as follows.

E (Excellent): 8 or more

G (Good): 4 to 7

F (Fair): 3 or less

[Production of Golf Ball]

1. Production of Spherical Core Spherical Cores No. 1 to 8

The center rubber compositions were prepared by kneading the materials with a kneading roll according to the formulations shown in Table 1. The spherical centers were formed by heat-pressing the center rubber compositions shown in Table 1 at 170° C. for 15 minutes in upper and lower molds, each having a hemispherical cavity. Then, the half shells were formed from the outer core rubber compositions shown in Table 1. The center was covered with two of the half shells. The center together with the half shells was heat-pressed at 142° C. for 20 minutes and further heat-pressed at 160° C. for 10 minutes in upper and lower molds, each having a hemispherical cavity, to obtain the spherical core. It is noted that, in Table 1, the amount of barium sulfate was adjusted such that the golf ball had a mass of 45.6 g and the center had a density identical to the outer core.

Spherical Core No. 9

The rubber composition was prepared by kneading the materials with a kneading roll according to the formulation shown in Table 1. The single-layered spherical core was formed by heat-pressing the rubber compositions at 150° C. for 20 minutes in upper and lower molds, each having a hemispherical cavity. It is noted that, in Table 1, the amount of barium sulfate was adjusted such that the golf ball had a mass of 45.6 g.

Spherical Core No. 10

The rubber composition was prepared by kneading the materials with a kneading roll according to the formulation shown in Table 1. The single-layered spherical core was formed by heat-pressing the rubber compositions at 142° C. for 20 minutes and further heat-pressed at 160° C. for 10 minutes in upper and lower molds, each having a hemispherical cavity. It is noted that, in Table 1, the amount of barium sulfate was adjusted such that the golf ball had a mass of 45.6 g.

TABLE 1 Core No. 1 2-1 2-2 3 4-1 4-2 5 6 7 8 9 10 Center Formulation Polybutadiene rubber 100 100 100 100 100 100 100 100 100 100 (parts by Antioxidant — — — — — — 0.13 0.1 mass) Zinc acrylate 28 27.5 26 28.5 24.5 26.5 27 25.5 35 44 Zinc oxide 5 5 5 5 5 5 5 5 12 5 Barium sulfate * * * * * * * * * * DCP 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.8 0.7 DPDS 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 — 0.5 PBDS — — — — — — — — 0.3 — 2-Thionaphthol — — — — — — — — 0.1 — Benzoic acid — — — — — — — — 2 — Curing First Temperature (° C.) 170 170 170 170 170 170 170 170 150 142 conditions stage Time (min) 15 15 15 15 15 15 15 15 20 20 Second Temperature (° C.) — — — — — — — — — 160 stage Time (min) — — — — — — — — — 10 Diameter (mm) 24 24 24 24 24 24 24 24 39.7 39.7 Outer Formulation Polybutadiene rubber 100 100 100 100 100 100 100 100 core (parts by Zinc acrylate 47.5 45.5 43.5 50.0 43.0 44.0 44.5 42.0 mass) Zinc oxide 5 5 5 5 5 5 5 5 Barium sulfate * * * * * * * * DPDS 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 DCP 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 Curing First Temperature (° C.) 142 142 142 142 142 142 142 142 conditions stage Time (min) 20 20 20 20 20 20 20 20 Second Temperature (° C.) 160 160 160 160 160 160 160 160 stage Time (min) 10 10 10 10 10 10 10 10 Thickness (mm) 8.2 7.9 7.3 7.3 6.9 7.3 8.1 7.8 7.6 7.3 Core Diameter (mm) 40.3 39.7 38.5 38.5 37.7 38.5 40.1 39.5 39.1 38.5 39.7 39.7 Center hardness (Shore C) 62.0 61.5 61.5 60.0 63.0 63.0 58.0 60.5 61.0 59.5 53.0 60.0 Surface hardness (Shore C) 84.0 83.5 83.5 82.0 85.0 85.0 80.0 82.5 83.0 81.5 79.0 70.0 Hardness difference (surface hardness- center hardness) (Shore C) 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 26.0 10.0 *Appropriate amount

Polybutadiene rubber: “BR730 (high cis-polybutadiene)” available from JSR Corporation

Antioxidant: “H-BHT” (dibutylhydroxytoluene) available from Honshu Chemical Industry Co., Ltd.

Zinc acrylate: “ZN-DA90S” available from Nisshoku Techno Fine Chemical Co., Ltd.

Zinc oxide: “Ginrei R” available from Toho Zinc Co., Ltd.

Barium sulfate: “Barium Sulfate BD” available from Sakai Chemical Industry Co., Ltd.

DCP: “Percumyl (registered trademark) D (dicumyl peroxide)” available from NOF Corporation

DPDS: Diphenyl disulfide available from Sumitomo Seika Chemicals Co., Ltd.

PBDS: bis(pentabromophenyl)persulfide available from Kawaguchi Chemical Industry Co., Ltd.

2-Thionaphthol: available from Zhejiang shou & Fu Chemical Co., Ltd.

Benzoic acid: available from Emerald Kalama Chemical, LLC

2. Preparation of intermediate layer composition and cover composition

The materials having the formulations shown in Tables 2 and 3 were mixed with a twin-screw kneading extruder to prepare the intermediate layer composition and cover composition in a pellet form. The extruding conditions were a screw diameter of 45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and the mixtures were heated to 160° C. to 230° C. at the die position of the extruder.

TABLE 2 Intermediate layer composition No. a b c d e Formulation Surlyn 8150 50 — — 50 — (parts by Surlyn 9150 — — — 50 — mass) Himilan 1605 — — 44 — 47 Himilan AM7329 50 31 50 — 50 Himilan AM7337 — 24 — — — Nucrel N1050H — 16 — — — Rabalon T3221C — 29 6 — 3 Titanium dioxide 3 3 3 3 3 Properties Material hardness 68 43 63 70 63 (Shore D)

Surlyn (registered trademark) 8150: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from E.I. du Pont de Nemours and Company

Surlyn 9150: zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from E.I. du Pont de Nemours and Company

Himilan (registered trademark) 1605: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan AM7329: zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan AM7337: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Nucrel (registered trademark) N1050H: ethylene-methacrylic acid copolymer available from Du Pont-Mitsui Polychemicals Co., Ltd.

Rabalon (registered trademark) T3221C: thermoplastic styrene elastomer available from Mitsubishi Chemical Corporation

TABLE 3 Cover composition No. A B C D Formulation Elastollan NY80A — 100 — — (parts by Elastollan NY84A 100 — — — mass) Elastollan NY88A — — 100 — Elastollan NY97A — — — 100 TINUVIN 770 0.2 0.2 0.2 0.2 Titanium oxide 4 4 4 4 Ultramarine blue 0.04 0.04 0.04 0.04 Properties Material hardness 84 80 88 97 (Shore A) Material hardness 31 27 36 47 (Shore D)

Elastollan (registered trademark) NY80A: thermoplastic polyurethane elastomer (Shore A hardness: 80) available from BASF Japan Ltd.

Elastollan NY84A: thermoplastic polyurethane elastomer (Shore A hardness: 84) available from BASF Japan Ltd.

Elastollan NY88A: thermoplastic polyurethane elastomer (Shore A hardness: 88) available from BASF Japan Ltd.

Elastollan NY97A: thermoplastic polyurethane elastomer (Shore A hardness: 97) available from BASF Japan Ltd.

TINUVIN (registered trademark) 770: hindered amine light stabilizer (bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate) available from BASF Japan Ltd.

3. Molding of Intermediate Layer

The intermediate layer composition obtained above was directly injection molded on the spherical core obtained as described above to form the intermediate layer covering the spherical core. Upper and lower molds for molding have a hemispherical cavity and a retractable hold pin for holding the spherical core. When molding the intermediate layer, the hold pin was protruded to hold the spherical core after the spherical core was charged, the intermediate layer composition was charged into the mold and cooled, and then the spherical body was ejected from the mold.

4. Formation of Reinforcing Layer

A reinforcing layer composition (trade name “Polin (registered trademark) 750LE” available from Shinto Paint Co., Ltd.) containing a two-component curing type epoxy resin as a base resin was prepared. The base material contained 30 parts by mass of bisphenol A type solid epoxy resin and 70 parts by mass of solvent. The curing agent contained 40 parts by mass of modified polyamide amine, 5 parts by mass of titanium dioxide and 55 parts by mass of solvent. The mass ratio of the base material to the curing agent was 1/1. The reinforcing layer composition was applied to the surface of the intermediate layer with an air gun, and kept for 12 hours at the atmosphere of 23° C. to form the reinforcing layer. The thickness of the reinforcing layer was 7 μm.

5. Molding of Cover

The cover composition in a pellet form was charged into each of the depressed part of the lower mold for molding half shells, and then a pressure was applied to mold the half shells. The compression molding was conducted under the conditions of a molding temperature of 170° C., a molding time of 5 minutes and a molding pressure of 2.94 MPa. The spherical body having the reinforcing layer formed thereon was concentrically covered with two of the half shells. The spherical body and the half shells were charged into a final mold provided with a plurality of pimples on the surface of the cavity, and compression molding was conducted to form the cover. The compression molding was conducted under the conditions of a molding temperature of 150° C., a molding time of 2 minutes and a molding pressure of 9.8 MPa. A plurality of dimples having an inverted form of the pimples were formed on the cover.

6. Preparation of Polyol Composition

6.1 Polyol Composition No. 1 (Urethane Polyol)

As a first polyol component, polytetramethylene ether glycol (PTMG, number average molecular weight: 650) and trimethylolpropane (TMP) were dissolved in a solvent (toluene and methyl ethyl ketone). The molar ratio (PTMG:TMP) was 1.8:1.0. Then, dibutyltin dilaurate which was used as a catalyst was added therein in an amount of 0.1 mass % with respect to the total amount of the base material. While keeping the temperature of the obtained polyol solution at 80° C., isophorone diisocyanate (IPDI) which was used as a first polyisocyanate component was added dropwise to the polyol solution and mixed. It is noted that the molar ratio (NCO/OH) of the NCO group in the polyisocyanate component to the OH group in the polyol component was 0.6. After adding of isophorone diisocyanate, stirring was continued until the isocyanate group no longer existed. Then, the reaction liquid was cooled to the room temperature to obtain the urethane polyol (amount of solid component: 30 mass %). In the obtained polyol composition No. 1, the amount of PTMG was 67 mass %, the hydroxyl value of the solid component was 67.4 mgKOH/g, and the urethane polyol had a weight average molecular weight of 4867.

6.2 Polyol Composition No. 2 (Urethane Polyol)

As a first polyol component, polytetramethylene ether glycol (PTMG, number average molecular weight: 1000) and trimethylolpropane (TMP) were dissolved in a solvent (toluene and methyl ethyl ketone). The molar ratio (PTMG:TMP) was 1.8:1.0. Then, dibutyltin dilaurate which was used as a catalyst was added therein in an amount of 0.1 mass % with respect to the total amount of the main material. While keeping the temperature of the obtained polyol solution at 80° C., isophorone diisocyanate (IPDI) which was used as a first polyisocyanate component was added dropwise to the polyol solution and mixed. It is noted that the molar ratio (NCO/OH) of the NCO group in the polyisocyanate component to the OH group in the polyol component was 0.6. After adding of isophorone diisocyanate, stirring was continued until the isocyanate group no longer existed. Then, the reaction liquid was cooled to the room temperature to obtain the urethane polyol (amount of solid component: 30 mass %). In the obtained polyol composition No. 2, the amount of PTMG was 76 mass %, the hydroxyl value of the solid component was 49.5 mgKOH/g, and the urethane polyol had a weight average molecular weight of 6624.

6.3 Polyol Composition No. 3 (Polyrotaxane Composition)

50 parts by mass of a polyrotaxane (“SeRM (registered trademark) super polymer SH3400P (a polyrotaxane having a cyclodextrin, at least a part of hydroxyl groups thereof being modified with a caprolactone chain via —O—C3H6-O— group, a linear molecule of polyethylene glycol and a blocking group of an adamantyl group; molecular weight of linear molecule: 35,000, hydroxyl value: 72 mg KOH/g, total molecular weight of polyrotaxane: 700,000 in weight average molecular weight)” available from Advanced Softmaterials Inc.), 28 parts by mass of polycaprolactone polyol (“Placcel (registered trademark) 308” available from Daicel Chemical Industries, Ltd.), 22 parts by mass of a vinyl chloride-vinyl acetate-vinyl alcohol copolymer (“Solbin (registered trademark) AL (hydroxyl value: 63.4 mg KOH/g) ” available from Nissin Chemical Industry Co., Ltd.), 0.1 part by mass of a modified silicone (DBL-C31 available from Gelest, Inc.), 0.01 part by mass of dibutyltin dilaurate, and 100 parts by mass of a solvent (a mixed solvent of xylene/methylethyl ketone=70/30 (mass ratio)) were mixed to prepared the polyol composition No. 3.

7. Preparation of Polyisocyanate Composition

7-1. Polyisocyanate Composition No. 1

30 parts by mass of an isocyanurate of hexamethylene diisocyanate (Duranate (registered trademark) TKA-100 (NCO amount: 21.7 mass %) available from Asahi Kasei Chemicals Corporation), 30 parts by mass of a biuret-modified product of hexamethylene diisocyanate (Duranate 21S-75E (NCO amount: 15.5 mass %) available from Asahi Kasei Chemicals Corporation), and 40 parts by mass of an isocyanurate of isophorone diisocyanate (Desmodur (registered trademark) Z 4470 (NCO amount: 11.9 mass %) available from Bayer company) were mixed. As a solvent, a mixed solvent of methyl ethyl ketone, n-butyl acetate and toluene was added therein to adjust the concentration of the polyisocyanate component as 60 mass %.

7-2. Polyisocyanate Composition No. 2

100 parts by mass of a biuret-modified product of hexamethylene diisocyanate (Duranate 21S-75E (NCO amount: 15.5 mass %) available from Asahi Kasei Chemicals Corporation), and 100 parts by mass of methyl ethyl ketone were mixed.

8. Paint

The paint formulations are shown in Table 4.

TABLE 4 Paint No. 1 2 3 4 5 6 7 Formulation Polyol composition No. 1 No. 1 No. 1 No. 1 No. 1 No. 2 No. 3 No. Polyisocyanate No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 2 composition No. Mass ratio (base 100/9.2 100/17 100/24 100/29 100/38.6 100/19.2 100/17 material/curing agent) Molar ratio NCO/OH) 0.38/1.0 0.7/1.0 1.0/1.0 1.2/1.0 1.6/1.0 1.04/1.0 1.03/1.0 Properties of 10% Elastic modulus 7.9 100.0 150.0 186.4 274.9 75.0 8.2 paint film (kgf/cm²) 50% Elastic modulus 14.3 118.5 178.8 209.4 250.6 110.0 32.4 (kgf/cm²) 100% Elastic modulus 21.9 161.1 240.0 278.6 — 163.0 — (kgf/cm²) Maximum elastic 119.8 195.0 276.7 337.1 293.3 416.0 40.2 modulus (kgf/cm²) Maximum elongation 260.5 140.0 114.5 120.7 76.4 196.0 51.4 (%)

9. Formation of Paint

The surface of the golf ball body obtained above was subjected to a sandblast treatment, and a mark was formed thereon. Then, the paint was applied to the golf ball body with a spray gun, and the paint was dried in an oven of 40° C. for 24 hours to obtain the golf ball having a mass of 45.6 g. The golf ball body was placed in a rotating member provided with three prongs, the rotating member was allowed to rotate at 300 rpm, and application of the paint was conducted by spacing a spray distance (7 cm) between the air gun and the golf ball body while moving the air gun in an up and down direction. Application of the paint was conducted under the air gun spraying conditions of a spraying air pressure: 0.15 MPa, a compressed air tank pressure: 0.10 MPa, a painting time for one application: 1 second, an atmosphere temperature: 20° C. to 27° C., and an atmosphere humidity: 65% or less. It is noted that after the inner layer paint sprayed on the golf ball body was completely dried, the spraying of the outer layer paint was conducted. Evaluation results of the obtained golf balls are shown in Tables 5 and 6.

TABLE 5 Golf ball No. 1 2 3 4 5 6 7 8 Core No. 3 3 3 3 3 3 3 2-1 Diameter (mm) 38.5 38.5 38.5 38.5 38.5 38.5 38.5 39.7 Center hardness (Shore C) 60.0 60.0 60.0 60.0 60.0 60.0 60.0 61.5 Surface hardness (Shore C) 82.0 82.0 82.0 82.0 82.0 82.0 82.0 83.5 Hardness difference HDc (Shore C) 22 22 22 22 22 22 22 22 Intermediate Formulation a a a a a a a a layer Thickness Tm (mm) 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.0 Material hardness Hm (Shore D) 68 68 68 68 68 68 68 68 Volume Vm of 8087 8087 8087 8087 8087 8087 8087 5205 intermediate layer (mm³) Cover Formulation A A A A A A A A Thickness Tc (mm) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Material hardness 31 31 31 31 31 31 31 31 Hc (Shore D) Phantom volume Vc of cover (mm³) 2797 2797 2797 2797 2797 2797 2797 2797 Paint Inner Paint No. 2 3 4 5 2 4 4 4 film layer 10% Elastic 100.0 150.0 186.4 274.9 100.0 186.4 186.4 186.4 modulus (kgf/cm²) Thickness (μm) 10 10 10 10 10 10 10 10 Outer Paint No. 6 6 6 6 1 1 7 6 layer 10% Elastic 75.0 75.0 75.0 75.0 7.9 7.9 8.2 75.0 modulus (kgf/cm²) Thickness (μm) 10 10 10 10 10 10 10 10 10% Elastic modulus 25 75 111.4 199.9 92.1 178.5 178.2 111.4 difference (kgf/cm²) (HDc*Hm)/(Hc*Tc) 96.5 96.5 96.5 96.5 96.5 96.5 96.5 96.5 (Vm*Hm)/(Vc*Hc) 6.34 6.34 6.34 6.34 6.34 6.34 6.34 4.08 Evaluation Compression deformation amount (mm) 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 Durability G G G G G G G G W#1 Ball initial velocity (m/s) 73.3 73.3 73.3 73.3 73.3 73.3 73.3 73.4 Spin rate (rpm) 2500 2500 2500 2500 2500 2500 2500 2450 Flight distance (yd) 290.0 290.0 290.0 290.0 290.0 290.0 290.0 291.0 AP-DRY Spin rate (rpm) 6470 6480 6500 6510 6420 6430 6400 6510 Coefficient of friction 0.613 0.602 0.593 0.581 0.635 0.627 0.633 0.581 AP-WET Spin rate (rpm) 3760 4000 4100 4190 3800 4050 4000 4190 Coefficient of friction 0.174 0.192 0.200 0.210 0.175 0.198 0.195 0.210 Spin rate ratio (WET/DRY) 0.58 0.62 0.63 0.64 0.59 0.63 0.63 0.64 Shot feeling G G G G E E E G Golf ball No. 9 10 11 12 13 14 15 Core No. 2-2 8 5 6 2-1 2-1 9 Diameter (mm) 38.5 38.5 40.1 39.5 39.7 39.7 39.7 Center hardness (Shore C) 61.5 59.5 58.0 60.5 61.5 61.5 53.0 Surface hardness (Shore C) 83.5 81.5 80.0 82.5 83.5 83.5 79.0 Hardness difference HDc (Shore C) 22 22 22 22 22 22 26 Intermediate Formulation c d a a a a e layer Thickness Tm (mm) 1.6 1.6 1.0 1.0 1.0 1.0 1.0 Material hardness Hm (Shore D) 60 70 68 68 68 68 63 Volume Vm of 8087 8087 5308 5154 5205 5205 5205 intermediate layer (mm³) Cover Formulation A A A A B C B Thickness Tc (mm) 0.5 0.5 0.3 0.6 0.5 0.5 0.5 Material hardness 31 31 31 31 27 36 27 Hc (Shore D) Phantom volume Vc of cover (mm³) 2797 2797 1694 3341 2797 2797 2797 Paint Inner Paint No. 4 4 4 4 4 4 4 film layer 10% Elastic 186.4 186.4 186.4 186.4 186.4 186.4 186.4 modulus (kgf/cm²) Thickness (μm) 10 10 10 10 10 10 10 Outer Paint No. 6 6 6 6 6 6 6 layer 10% Elastic 75.0 75.0 75.0 75.0 75.0 75.0 75.0 modulus (kgf/cm²) Thickness (μm) 10 10 10 10 10 10 10 10% Elastic modulus 111.4 111.4 111.4 111.4 111.4 111.4 111.4 difference (kgf/cm²) (HDc*Hm)/(Hc*Tc) 85.2 99.4 160.9 80.4 110.8 83.1 121.3 (Vm*Hm)/(Vc*Hc) 5.60 6.53 6.87 3.38 4.69 3.51 4.34 Evaluation Compression deformation amount (mm) 2.2 2.2 2.2 2.2 2.2 2.2 2.2 Durability G G G G G G G W#1 Ball initial velocity (m/s) 73.2 73.3 73.5 73.3 73.4 3.4 72.9 Spin rate (rpm) 2550 2450 2350 2550 2500 2350 2350 Flight distance (yd) 289.0 290.5 292.5 289.5 290.5 292.0 289.5 AP-DRY Spin rate (rpm) 6550 6450 6360 6610 6580 6410 6480 Coefficient of friction 0.500 0.615 0.660 0.400 0.450 0.635 0.600 AP-WET Spin rate (rpm) 4150 4050 3990 4290 4120 4290 4100 Coefficient of friction 0.205 0.198 0.190 0.220 0.203 0.220 0.200 Spin rate ratio (WET/DRY) 0.63 0.63 0.63 0.65 0.63 0.67 0.63 Shot feeling E G G E E G E

TABLE 6 Golf ball No. 16 17 18 19 20 21 22 23 24 25 Core No. 3 3 3 3 2-1 1 4-1 4-2 7 10 Diameter (mm) 38.5 38.5 38.5 38.5 39.7 40.3 37.7 38.5 39.1 39.7 Center hardness (Shore C) 60.0 60.0 60.0 60.0 61.5 62.0 63.0 63.0 61.0 60.0 Surface hardness (Shore C) 82.0 82.0 82.0 82.0 83.5 84.0 85.0 85.0 83.0 70.0 Hardness difference HDc (Shore C) 22 22 22 22 22 22 22 22 22 10 Intermediate Formulation a a a a a a a b a e layer Thickness Tm (mm) 1.6 1.6 1.6 1.6 1.0 0.7 2.0 1.6 1.0 1.0 Material hardness Hm (Shore D) 68 68 68 68 68 68 68 43 68 63 Volume Vm of intermediate layer 8087 8087 8087 8087 5205 3697 9911 8087 5053 5205 (mm³) Cover Formulation A A A A D A A A A B Thickness Tc (mm) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.8 0.5 Material hardness Hc (Shore D) 31 31 31 31 47 31 31 31 31 27 Phantom volume Vc of cover (mm³) 2797 2797 2797 2797 2797 2797 2797 2797 4413 2797 Paint film Inner Paint No. 6 4 1 7 4 4 4 4 4 4 layer 10% Elastic modulus 75.0 186.4 7.9 8.2 186.4 186.4 186.4 186.4 186.4 186.4 (kgf/cm²) Thickness (μm) 10 10 10 10 10 10 10 10 10 10 Outer Paint No. 6 4 1 7 6 6 6 6 6 6 layer 10% Elastic modulus 75.0 186.4 7.9 8.2 75.0 75.0 75.0 75.0 75.0 75.0 (kgf/cm²) Thickness (μm) 10 10 10 10 10 10 10 10 10 10 10% Elastic modulus difference 0 0 0 0 111.4 111.4 111.4 111.4 111.4 111.4 (kgf/cm²) (HDc*Hm)/(Hc*Tc) 96.5 96.5 96.5 96.5 63.7 96.5 96.5 61.0 60.3 46.7 (Vm*Hm)/(Vc*Hc) 6.34 6.34 6.34 6.34 2.69 2.90 7.77 4.01 2.51 4.34 Compression deformation amount 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 (mm) Durability G G G G G P G G G G W#1 Ball initial velocity (m/s) 73.3 73.3 73.3 73.3 73.4 73.6 73.1 73 73.2 73.1 Spin rate (rpm) 2500 2500 2500 2500 2250 2400 2550 2650 2700 2650 Flight distance (yd) 290.0 290.0 290.0 290.0 293.0 292.5 288.5 287.0 287.5 288.5 AP-DRY Spin rate (rpm) 6440 6520 6400 6380 6310 6510 6310 6600 6710 6430 Coefficient of friction 0.621 0.500 0.642 0.655 0.670 0.581 0.670 0.420 0.150 0.627 AP-WET Spin rate (rpm) 3600 4420 3380 3210 4260 4190 3990 4200 4390 4050 Coefficient of friction 0.168 0.225 0.152 0.150 0.217 0.210 0.196 0.212 0.223 0.198 Spin rate ratio (WET/DRY) 0.56 0.68 0.53 0.50 0.68 0.64 0.63 0.64 0.65 0.63 Shot feeling G F E E F G G E E E

The golf balls No. 1 to 15 are the cases that a difference (Min-Mout) between two layers of the paint film is 25 kgf/cm2 or more, the intermediate layer has a thickness Tm of 0.75 mm or more, the cover has a material hardness Hc of 40 or less in Shore D hardness, and the construction of the golf ball body satisfies the mathematical formulae (1) and (2). These golf balls No. 1 to 15 travel a great flight distance on driver shots, have a high spin rate on approach shots under a dry condition and a wet condition, and show an excellent shot feeling and durability.

The golf balls No. 16 to 19 are the cases that the difference (Min-Mout) is less than 25 kgf/cm2. These golf balls No. 16 to 19 have a low spin rate on approach shots under a wet condition, or show an inferior shot feeling. The golf ball No. 20 is the case that the cover has a material hardness Hc of more than 40 in Shore D hardness, and the ratio ((Vm×Hm)/(Vc×Hc)) is 3.0 or less. The golf ball No. 20 has a low spin rate on approach shots under a dry condition, and shows an inferior shot feeling as well. The golf ball No. 21 is the case that the intermediate layer has a thickness Tm of less than 0.75 mm, and thus shows an inferior durability. The golf ball No. 22 is the case that the ratio ((Vm×Hm)/(Vc×Hc)) is 7.5 or more, and thus has a low spin rate on approach shots under a dry condition. The golf balls No. 23 to 25 are the cases that the ratio ((HDc×Hm)/(Hc×Tc)) is 63 or less, and thus have a high spin rate on driver shots.

This application is based on Japanese patent application No. 2015-247257 filed on Dec. 18, 2015, the contents of which are hereby incorporated by reference. 

1. A golf ball comprising a golf ball body and a paint film formed on a surface of the golf ball body, wherein the golf ball body comprises a spherical core, an intermediate layer covering the spherical core and a cover covering the intermediate layer, and the paint film has a multi-layered construction composed of two or more layers, and wherein a difference (M_(in)-M_(out)) between a 10% elastic modulus (M_(in)) of an innermost layer of the paint film and a 10% elastic modulus (M_(out)) of an outermost layer of the paint film is 65 kgf/cm² or more, the 10% elastic modulus (M_(out)) ranges from 5 kgf/cm² to 15 kgf/cm², a ratio (T_(out)/T_(in)) of a thickness (T_(out)) of the outermost layer of the paint film to a thickness (T_(in)) of the innermost layer of the paint film ranges from 0.2 to 1.0.
 2. The golf ball according to claim 1, wherein the intermediate layer has a thickness Tm of 0.75 mm or more.
 3. The golf ball according to claim 1, wherein the cover has a material hardness Hc of 40 or less in Shore D hardness.
 4. The golf ball according to claim 1, wherein the intermediate layer has a material hardness Hm of 55 or more in Shore D hardness.
 5. The golf ball according to claim 1, wherein a hardness difference (surface hardness-center hardness) HDc (Shore C hardness) between a surface hardness and a center hardness of the spherical core, a material hardness Hm (Shore D hardness) of the intermediate layer, a thickness Tc (mm) of the cover, and a material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (1); (HDc×Hm)/(Hc×Tc)>63   Mathematical formula (1).
 6. The golf ball according to claim 1, wherein a volume Vm (mm³) and a material hardness Hm (Shore D hardness) of the intermediate layer, a phantom volume Vc (mm³) and a material hardness Hc (Shore D hardness) of the cover satisfy the following mathematical formula (2);
 3. 0<(Vm×Hm)/(Vc×Hc)<7.5   Mathematical formula (2).
 7. The golf ball according to claim 1, wherein the 10% elastic modulus (M_(in)) of the innermost layer of the paint film is 100 kgf/cm² or more.
 8. The golf ball according to claim 1, wherein the innermost layer of the paint film has a thickness (T_(in)) ranging from 5 μm to 30 μm, and the outermost layer of the paint film has a thickness (T_(out)) ranging from 5 μm to 30 μm.
 9. The golf ball according to claim 1, wherein the intermediate layer has a thickness Tm of 0.8 mm or more.
 10. The golf ball according to claim 5, wherein the hardness difference HDc ranges from 15 to 40 in Shore C hardness.
 11. The golf ball according to claim 5, wherein the surface hardness of the spherical core ranges from 75 to 90 in Shore C hardness.
 12. The golf ball according to claim 5, wherein the center hardness of the spherical core ranges from 45 to 70 in Shore C hardness.
 13. The golf ball according to claim 1, wherein the spherical core is a dual-layered core composed of a spherical center and an outer core covering the spherical center.
 14. The golf ball according to claim 6, wherein the volume Vm of the intermediate layer ranges from 3900 mm³ to 9500 mm³.
 15. The golf ball according to claim 5, wherein the thickness Tc of the cover ranges from 0.2 mm to 0.7 mm.
 16. The golf ball according to claim 6, wherein the phantom volume Vc of the cover ranges from 1100 mm³ to 3900 mm³.
 17. The golf ball according to claim 7, wherein the 10% elastic modulus (M_(in)) of the innermost layer of the paint film is 500 kgf/cm² or less.
 18. The golf ball according to claim 1, wherein a base resin constituting the paint film includes a polyurethane resin. 